10 ms·

How Do You Get a Train Moving?

95 points by zw123456 12y ago

kimonos 12y ago
Cool! Will share this to my son coz he loves trains..
NN88 12y ago
ALWAYS wondered this...
DINKDINK 12y ago
This is a very poorly written article (I'm amazed this is written by a physics professor). Static and kinetic friction is only a resistive force to a train if there is slippage between the wheels and the track. -Assuming a no slip condition at the wheels, the friction will rotate the wheels not drag on the train/car. -The reason why there is a gap/space in the train couplings is a tolerance/GD&T issue. -Engine torque is almost always higher at lower speeds http://image.circletrack.com/f/enginetech/ctrp_1003_nascar_sprint_cup_engine/27509422/ctrp_1003_21_z%2Bnascar_sprint_cup_engine%2Bdyno_sheet.jpg http://image.circletrack.com/f/enginetech/ctrp_1003_nascar_s... (this is why you have to down-shift when climbing a hill) and is what gets a vehicle moving. Top commenter got it right:"The locomotive is accelerating itself and the cars, not overcoming the friction in the bearings. Axle bearings have pretty low friction (at least negligible compared to the inertia of a loaded car); if they didn't, they would get hot, the grease would melt"
- mankyd 12y ago
  I don't quite see what's amiss. > Static and kinetic friction is only a resistive force to a train if there is slippage between the wheels and the track. No, static friction affects the train _until_ it starts moving. If there is any slippage or movement or anything, you've moved into the realm of kinetic friction, whether it be between the wheels and the track, or the axle and its bearings. > Axle bearings have pretty low friction ... if they didn't, they would get hot, the grease would melt Axle bearings have low _kinetic_ friction. They can, in theory, have a much higher static friction, and this is what needs to be overcome to get the train moving.
  - msandford 12y ago
    It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller bearings. Static friction is when things are not sliding and kinetic friction is when they are sliding. Since the cars have roller bearings (not bushings) the wheel doesn't slide relative to the track and the axle doesn't slide relative to the axle truck. There might be slightly more start-up torque to get a bearing moving but that's more as a result of the static friction of the seals or the roller cage or the grease. The reason that bearings work so well is that they DON'T slide but rather roll. They're an ingenious hack around the idea that you have to pay the price somewhere for friction. Before they existed everyone bitched about friction but "hey what can you do?!" Afterwards it seemed obvious but the world spent many thousands of years with wheels and no bearings.
    - tanzam75 12y ago
      > It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller bearings. Well, modern trains don't have cabooses either. So the scenario wouldn't even come up. But suppose that this is a train from 1940, with a caboose, with journal bearings instead of roller bearings, and with a steam engine. Can it start? I wonder if maybe this was a legitimate problem from the steam era, which led to the procedure to compress the train and the start it one car at a time. It's conceivable that we'd keep passing along the received wisdom, even after roller bearings and electric motors changed the picture.
- fryguy 12y ago
  Fundamentally, static friction is the force that's going to be preventing the train from starting to move. If you had a semi-truck in neutral on a flat surface, I don't think you would be able to push it. The wheels aren't slipping on the ground, but rather it must be in the bearings. The problem is F=ma. In this case m is large.
  - jessriedel 12y ago
    I've pushed a school bus on flat ground, and I've pushed it uphill with 3 other. The friction in the bearings is negligible.
- msisk6 12y ago
  Slack is something railroads would love to get rid of. If the slack isn't managed properly when starting a train the jerk can rip the coupler knuckle right off and split the train (which brings everything to a stop when the brakes automatically apply when the air supply hose is separated). I've seen many a train startup on a grade with all the slack stretched out and wrapped around many degrees of curvature: modern diesel-electric locomotives have tremendous tractive effort right from a standstill. These modern locomotives have computer-controlled wheel slip systems, sand dispensers (that inject sand between the wheel and the rail to increase friction), and active steering trucks. And for the past decade or so locomotives have switched from DC traction motors to phased AC (which don't have high-current brushes that can melt at high-load, low-speed). All US railroads have mandated low-friction roller bearings in everything since the 60s or 70s, too. I'd imagine the railway engineering field has all the physics of this stuff well-documented since the basics of railroad operation hasn't really changed in 100+ years.
  - jessriedel 12y ago
    I can't understand what your reply has to do with DINKDINK's comment.
    - andrewflnr 12y ago
      Possibly they meant to reply to https://news.ycombinator.com/item?id=7837782 https://news.ycombinator.com/item?id=7837782
      - deleted 12y ago
        [deleted]
    - VLM 12y ago
      I would have been a 4th generation RR employee, if the RR hadn't gone bankrupt. Got to spend all kinds of time as a kid in rail yards going stuff that would probably be illegal for everyone involved now a days. This is before CCTV everywhere and extensive tracking. Anyway I'll try to match up first post comment/question with second post correct answer, if this reformat helps: Torque is higher at low speeds - No, with "modern" say post 1960 diesel-electric locos its constant. Or more accurately current limited to a max which is achievable at almost any reasonable low speed condition. Maybe a car analogy is all locos can "spin their tires" under any condition up to 45 MPH or so. There are no "econobox locomotives" or the equivalent. Peculiar spherical cow assumptions about no-slip conditions - No, its very complicated and as you'd suspect lots of anti-lock anti-skid technology actually started on trains before it went to cars, and trains are always slipping a little (not very much, and the optimum is not exactly zero just close to it) Slack is an intentional mfgr tolerance thing - No its something that could (almost) be eliminated and is a huge PITA under certain operating conditions. Its goal zero not goal 0.1% by design or whatever. Theoretically the bearings must be pretty good - yes indeed they are, awesome roller bearings required by regulation since the 60s If you provide specific examples of what doesn't make sense it would be possible to expand upon that specific topic.
- brc 12y ago
  >Engine torque is almost always higher at lower speeds This depends entirely on the engine - for most internal combustion engines, it's most definitely not the case. The reason you have to shift down when climbing a hill is to get the engine into a higher rev-range (compared to the wheels) than it currently is. With Nascar engines it may be that they are able to make a flat torque curve - probably because of the RPM restrictions. Most non-turbo passenger cars will make maximum torque in the mid-high end of their engine speed. Turbo/super charging assists in boosting torque output at lower engine RPM than in normally aspirated engines. The torque characteristics are a property of the bore/stroke relationship and the intake and exhaust tuning, as well as the camshaft timing. The reason we now have variable length intake manifolds and variable valve timing and lift on engines is to maintain the peak power operating combined with better torque at lower speeds, which aids drivability overall. Electric engines make maximum torque at 0 rpm, which is why most locomotives are diesel-electric, because the electric engine can make maximum RPM at standstil, while the diesel engines can be operated at higher RPM to create the most power.
  - Gravityloss 12y ago
    Even electric engines would benefit from gears. Thats because engine _power_ is higher at higher engine angular velocity power = torque * angular velocity so torque = power / angular velocity
    - eigenvector 12y ago
      You're assuming that torque remains constant at higher angular velocities, which is not true in most electric motor-drives.
      - Gravityloss 12y ago
        It doesn't stay constant. I didn't assume it stays constant either. Your maximum torque can be extracted by the right gear from the maximum _power_ peak. That means even with diminishing torque with angular velocity, if it diminishes only slowly, it's still worth going to higher angular velocities. Example: Say if you have a motor that has 100 Nm of torque at 100 radians/s, which is 10 kW of power, and 80 Nm of torque at 200 radians/s, which is 16 kW of power. And further, you specify that you need to have the maximum torque in the wheels at 10 radians/s. With a 10:1 gear ratio, when your wheels run at 10 radians/s, your engine runs at 100 radians/s and provides 10 kW. Thus the torque at the wheels is 10000W / 10 radians/s = 1000 Nm. With a 20:1 your engine runs at 200 radians/s and 16 kW and wheel torque is: 16000 W / 10 radians/s = 1800 Nm. More torque at the wheels with less torque at the engine! Of course, with a fixed ratio, the latter vehicle would be limited to a lower top speed. That's why we have gearboxes and variators etc. With an electric motor though that has better torque at lower speed, you can often try to design for a compromise so that you don't need a gearbox and might not even need a gear ratio at all. I used SI units since you get extraneous confusing factors otherwise. Here's an example of a brushless motor power vs rpm. You can get maximum torque for your selected angular velocity by operating at the max motor power and using the right gearing. http://www.mcpappyracing.com/images/dyno/chart_power.jpg http://www.mcpappyracing.com/images/dyno/chart_power.jpg
foobarian 12y ago
Of course modern trains have electric motors on each car, and engines are just giant diesel generators making the electricity...
- cbr 12y ago
  [citation needed] I'm not seeing anything about this; everything online says standard trains are still an active locomotive and passive cars.
  - ChuckMcM 12y ago
    http://en.wikipedia.org/wiki/Multiple-unit_train_control http://en.wikipedia.org/wiki/Multiple-unit_train_control covers the gist of it. One of the challenges of long trains is that you can't easily start them on a curve. As the front of the train is pulled it wants to 'straighten out' and that would pull mid-train cars off the track, Such trains if they don't use MU put locomotives both in the back and the front (and some times mid-train). The advantage of standardized MU is that your engine is simply a control center (especially if you're using power from overhead catenary wires) and any train of any length can "work" because the traction motors in every car are sufficient to push that car with some flexibility (some can push up to two additional 'dead load' cars.) But as others have pointed out, in the US the tracks are primarily straight, the existing traction motors in diesels system works well enough, and there is little incentive to change. That said, one of the options on the table for electrified CalTrain is MU equipped cars so that train sizes can be varied depending on commute load.
    - cbr 12y ago
      MU allows you to have multiple locomotives controlled together, but individual train cars still generally don't have traction motors. There are specialized traincars that have motors but not generators [1] but I don't think these were what foobarian was referring to. [1] http://en.wikipedia.org/wiki/Slug_(railroad) http://en.wikipedia.org/wiki/Slug_(railroad)
    - ubernostrum 12y ago
lutorm 12y ago
The article totally misses the fact that the static friction to overcome on the cars are a static torque not a force. The forces on the cars are acting on the coupling and at the wheel contact point, but the static friction to overcome is at the axle diameter. The torque arm is such that the force there is larger by the ratio of the wheel and axle radii. The larger the train wheels, the less effect the static friction will have.
js2 12y ago
Huh. http://en.wikipedia.org/wiki/Slack_action http://en.wikipedia.org/wiki/Slack_action "Loose coupling is necessary to enable the train to bend around curves and is an aid in starting heavy trains, since the application of the locomotive power to the train operates on each car in the train successively, and the power is thus utilized to start only one car at a time." An example at 2:20 in this video: http://www.youtube.com/watch?v=9D9zE6mju48&t=2m20s http://www.youtube.com/watch?v=9D9zE6mju48&t=2m20s
- refurb 12y ago
  So how do you get a locomotive started when the engine and cars are not on flat track? If there is any kind of incline, you'll have no slack in the couplings. Can you use the brakes somehow?
  - deleted 12y ago
    [deleted]
  - tempestn 12y ago
    I'm guessing you answered your own question. If they put the break on the last car, they could let the train compress backwards before starting. Naturally it would still be more work than starting from flat though, so obviously there would be an incline limit beyond which a given train couldn't start regardless.
    - refurb 12y ago
      Can the last car's brakes hold the weight of all the cars in front of it? Can the conductor apply each car's brakes independently? Interesting stuff!
      - tempestn 12y ago
        The original article stated that a train wasn't able to start because the caboose's brake was on, so presumably the answer to the second question is yes, at least for that car. Presumably the first question would depend on the slope of the incline (although if necessary other brakes could be engaged after their cars' couplings were compressed). Anyway, all just conjecture on my part!
femto 12y ago
It seems like cheating that the answer to "How Do YOU Get a Train Moving?" is "With a locomotive." I'd answer "with a crowbar". I used to work in a railway museum and when we didn't have an engine in steam we'd do our shunting with crowbars. The technique is to get a 6 foot long bar with a bent end. Wedge the point of the bent end between the tread and rail, so the "elbow" is resting an inch or two behind the wheel and the bar is up at a 45 degree angle, so forming a lever with a lot of mechanical advantage. Put all your weight on the bar. Eventually the car will start moving. As the car moves, release the bar, slide it forward and repeat. Eventually the car will be moving fast enough that you just run along beside it, sliding the bar forward and jacking up and down with your hands. One person can move a typical car on the flat, but if that's not enough, a person with bar on each wheel will get most things moving. Always have someone on the handbrake, as you will kill yourself if you try to stop the car with the bar!
- hueving 12y ago
  >and jacking up and down with your hands. Some of the best tools to jack with!
  - femto 12y ago
    Had to make it obvious! Once the car is moving, if you try to continue jacking by jumping on the bar you run the risk of going under the wheels. The wheels can easily slice you in half.
- samstave 12y ago
  Seems like all rail cars need a cyclical version of this, an elbow that can be lowered, and with a circular crank, be wedged and turned to produce forward motion. Or a wind-up where a spinning lever could compound effort onto an axle and drive the car forward...
- ohwp 12y ago
  Other answer: with a lot of model trains: https://www.youtube.com/watch?v=vC-dmKHiCnY#t=445 https://www.youtube.com/watch?v=vC-dmKHiCnY#t=445
epx 12y ago
I think that having to appeal to couplings' slack to be able to start a train was used in the steam era, because steam engines are constant-force machines (limited by steam pressure and cylinder area). Diesel-electric or electric machines can start stretched trains, even manuals of 1950 engines advise to start stretched. Starting slacked is dangerous because of the impacts on couples (current trains are much, much longer than steam-era trains).
rb2e 12y ago
From my father, who used to drive steam and diesel locomotives in the UK on a Heritage railway (2ft Gauge). He says strictly speaking, legally you're not allowed to reverse a train once passengers are inside without permission, but you can "rock" the locomotive forwards & backwards if you're stuck. Procedures may differ in the USA.
- VLM 12y ago
  Also the reasoning is you can derail a collection of (relatively) lightweight cars by pushing behind them with a very heavy engine, but its much harder to derail by pulling. Think of trying to keep a rope straight by pulling on it, vs trying to keep a rope straight by pushing on it. The passenger train rule is because its hard to kill people by tipping over coal cars especially when its a rare failure mode anyway. But you wouldn't risk it with cars full of passengers. (Oh and edited to add WRT switches, its subjectively 10 to 100 times harder to derail on a switch going one direction than the other, so again you'd risk backing a bunch of coal cars thru a switch but it would be pretty dumb to do it with cars full of passengers)
symmetricsaurus 12y ago
I am fairly certain that trains do not need the slack between cars to get moving. Like others have pointed out, trains seem to be perfectly capable of starting on inclines where there would be no slack. There is one assumption in the article that is certainly wrong: "...but it seems crazy to think that the train’s friction coefficient is 10 times more than the cars" This does not make any sense. The forces we are comparing is the friction between the rail and the wheels in the case of the engine and the friction in the bearings of the axles in the case of the cars. There is of course some frictional force from the deformation of the track and the wheel as well but as they are both quite rigid (being made of steel) it should not be very significant.
- hvm 12y ago
  The article has the physics completely wrong. Static friction is what opposes movement between two surfaces that are static next to each other and kinetic friction is what slows down movement between two surfaces. Static friction is always >= kinetic friction. This is explained very well in here (the top picture says it all): http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html Think about how when you start your car from a standstill you need to be careful about your clutch release but when the car moves even the tiniest bit you can even start in the second gear if you want. So, the engine car's wheels get kinetic friction (because they're rolling) and the other cars' wheels (which are stopped) are blocked by static friction. When you have slack and you move one car at a time you only need to beat the static friction of one car at a time. Even if the difference is 10%, if you have 10 cars and it starts to add up. I'm not saying slack is required for starting the train, just that there is an increased difficulty in doing so without slack.
  - hvm 12y ago
    Also, this is why doing a wheelspin is never a good idea when you have trouble starting your car/train. The traction when the wheels are blocked but the engine tries to move them is always a lot better than when they're spinning in place.
    - 12y ago

95 points by zw123456 12y ago

kimonos 12y ago

Cool! Will share this to my son coz he loves trains..

NN88 12y ago

ALWAYS wondered this...

DINKDINK 12y ago

This is a very poorly written article (I'm amazed this is written by a physics professor). Static and kinetic friction is only a resistive force to a train if there is slippage between the wheels and the track. -Assuming a no slip condition at the wheels, the friction will rotate the wheels not drag on the train/car. -The reason why there is a gap/space in the train couplings is a tolerance/GD&T issue. -Engine torque is almost always higher at lower speeds http://image.circletrack.com/f/enginetech/ctrp_1003_nascar_sprint_cup_engine/27509422/ctrp_1003_21_z%2Bnascar_sprint_cup_engine%2Bdyno_sheet.jpg http://image.circletrack.com/f/enginetech/ctrp_1003_nascar_s... (this is why you have to down-shift when climbing a hill) and is what gets a vehicle moving. Top commenter got it right:"The locomotive is accelerating itself and the cars, not overcoming the friction in the bearings. Axle bearings have pretty low friction (at least negligible compared to the inertia of a loaded car); if they didn't, they would get hot, the grease would melt"

mankyd 12y ago
I don't quite see what's amiss. > Static and kinetic friction is only a resistive force to a train if there is slippage between the wheels and the track. No, static friction affects the train _until_ it starts moving. If there is any slippage or movement or anything, you've moved into the realm of kinetic friction, whether it be between the wheels and the track, or the axle and its bearings. > Axle bearings have pretty low friction ... if they didn't, they would get hot, the grease would melt Axle bearings have low _kinetic_ friction. They can, in theory, have a much higher static friction, and this is what needs to be overcome to get the train moving.
- msandford 12y ago
  It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller bearings. Static friction is when things are not sliding and kinetic friction is when they are sliding. Since the cars have roller bearings (not bushings) the wheel doesn't slide relative to the track and the axle doesn't slide relative to the axle truck. There might be slightly more start-up torque to get a bearing moving but that's more as a result of the static friction of the seals or the roller cage or the grease. The reason that bearings work so well is that they DON'T slide but rather roll. They're an ingenious hack around the idea that you have to pay the price somewhere for friction. Before they existed everyone bitched about friction but "hey what can you do?!" Afterwards it seemed obvious but the world spent many thousands of years with wheels and no bearings.
  - tanzam75 12y ago
    > It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller bearings. Well, modern trains don't have cabooses either. So the scenario wouldn't even come up. But suppose that this is a train from 1940, with a caboose, with journal bearings instead of roller bearings, and with a steam engine. Can it start? I wonder if maybe this was a legitimate problem from the steam era, which led to the procedure to compress the train and the start it one car at a time. It's conceivable that we'd keep passing along the received wisdom, even after roller bearings and electric motors changed the picture.
fryguy 12y ago
Fundamentally, static friction is the force that's going to be preventing the train from starting to move. If you had a semi-truck in neutral on a flat surface, I don't think you would be able to push it. The wheels aren't slipping on the ground, but rather it must be in the bearings. The problem is F=ma. In this case m is large.
- jessriedel 12y ago
  I've pushed a school bus on flat ground, and I've pushed it uphill with 3 other. The friction in the bearings is negligible.
msisk6 12y ago
Slack is something railroads would love to get rid of. If the slack isn't managed properly when starting a train the jerk can rip the coupler knuckle right off and split the train (which brings everything to a stop when the brakes automatically apply when the air supply hose is separated). I've seen many a train startup on a grade with all the slack stretched out and wrapped around many degrees of curvature: modern diesel-electric locomotives have tremendous tractive effort right from a standstill. These modern locomotives have computer-controlled wheel slip systems, sand dispensers (that inject sand between the wheel and the rail to increase friction), and active steering trucks. And for the past decade or so locomotives have switched from DC traction motors to phased AC (which don't have high-current brushes that can melt at high-load, low-speed). All US railroads have mandated low-friction roller bearings in everything since the 60s or 70s, too. I'd imagine the railway engineering field has all the physics of this stuff well-documented since the basics of railroad operation hasn't really changed in 100+ years.
- jessriedel 12y ago
  I can't understand what your reply has to do with DINKDINK's comment.
  - andrewflnr 12y ago
    Possibly they meant to reply to https://news.ycombinator.com/item?id=7837782 https://news.ycombinator.com/item?id=7837782
    - deleted 12y ago
      [deleted]
  - VLM 12y ago
    I would have been a 4th generation RR employee, if the RR hadn't gone bankrupt. Got to spend all kinds of time as a kid in rail yards going stuff that would probably be illegal for everyone involved now a days. This is before CCTV everywhere and extensive tracking. Anyway I'll try to match up first post comment/question with second post correct answer, if this reformat helps: Torque is higher at low speeds - No, with "modern" say post 1960 diesel-electric locos its constant. Or more accurately current limited to a max which is achievable at almost any reasonable low speed condition. Maybe a car analogy is all locos can "spin their tires" under any condition up to 45 MPH or so. There are no "econobox locomotives" or the equivalent. Peculiar spherical cow assumptions about no-slip conditions - No, its very complicated and as you'd suspect lots of anti-lock anti-skid technology actually started on trains before it went to cars, and trains are always slipping a little (not very much, and the optimum is not exactly zero just close to it) Slack is an intentional mfgr tolerance thing - No its something that could (almost) be eliminated and is a huge PITA under certain operating conditions. Its goal zero not goal 0.1% by design or whatever. Theoretically the bearings must be pretty good - yes indeed they are, awesome roller bearings required by regulation since the 60s If you provide specific examples of what doesn't make sense it would be possible to expand upon that specific topic.
brc 12y ago
>Engine torque is almost always higher at lower speeds This depends entirely on the engine - for most internal combustion engines, it's most definitely not the case. The reason you have to shift down when climbing a hill is to get the engine into a higher rev-range (compared to the wheels) than it currently is. With Nascar engines it may be that they are able to make a flat torque curve - probably because of the RPM restrictions. Most non-turbo passenger cars will make maximum torque in the mid-high end of their engine speed. Turbo/super charging assists in boosting torque output at lower engine RPM than in normally aspirated engines. The torque characteristics are a property of the bore/stroke relationship and the intake and exhaust tuning, as well as the camshaft timing. The reason we now have variable length intake manifolds and variable valve timing and lift on engines is to maintain the peak power operating combined with better torque at lower speeds, which aids drivability overall. Electric engines make maximum torque at 0 rpm, which is why most locomotives are diesel-electric, because the electric engine can make maximum RPM at standstil, while the diesel engines can be operated at higher RPM to create the most power.
- Gravityloss 12y ago
  Even electric engines would benefit from gears. Thats because engine _power_ is higher at higher engine angular velocity power = torque * angular velocity so torque = power / angular velocity
  - eigenvector 12y ago
    You're assuming that torque remains constant at higher angular velocities, which is not true in most electric motor-drives.
    - Gravityloss 12y ago
      It doesn't stay constant. I didn't assume it stays constant either. Your maximum torque can be extracted by the right gear from the maximum _power_ peak. That means even with diminishing torque with angular velocity, if it diminishes only slowly, it's still worth going to higher angular velocities. Example: Say if you have a motor that has 100 Nm of torque at 100 radians/s, which is 10 kW of power, and 80 Nm of torque at 200 radians/s, which is 16 kW of power. And further, you specify that you need to have the maximum torque in the wheels at 10 radians/s. With a 10:1 gear ratio, when your wheels run at 10 radians/s, your engine runs at 100 radians/s and provides 10 kW. Thus the torque at the wheels is 10000W / 10 radians/s = 1000 Nm. With a 20:1 your engine runs at 200 radians/s and 16 kW and wheel torque is: 16000 W / 10 radians/s = 1800 Nm. More torque at the wheels with less torque at the engine! Of course, with a fixed ratio, the latter vehicle would be limited to a lower top speed. That's why we have gearboxes and variators etc. With an electric motor though that has better torque at lower speed, you can often try to design for a compromise so that you don't need a gearbox and might not even need a gear ratio at all. I used SI units since you get extraneous confusing factors otherwise. Here's an example of a brushless motor power vs rpm. You can get maximum torque for your selected angular velocity by operating at the max motor power and using the right gearing. http://www.mcpappyracing.com/images/dyno/chart_power.jpg http://www.mcpappyracing.com/images/dyno/chart_power.jpg

foobarian 12y ago

Of course modern trains have electric motors on each car, and engines are just giant diesel generators making the electricity...

cbr 12y ago
[citation needed] I'm not seeing anything about this; everything online says standard trains are still an active locomotive and passive cars.
- ChuckMcM 12y ago
  http://en.wikipedia.org/wiki/Multiple-unit_train_control http://en.wikipedia.org/wiki/Multiple-unit_train_control covers the gist of it. One of the challenges of long trains is that you can't easily start them on a curve. As the front of the train is pulled it wants to 'straighten out' and that would pull mid-train cars off the track, Such trains if they don't use MU put locomotives both in the back and the front (and some times mid-train). The advantage of standardized MU is that your engine is simply a control center (especially if you're using power from overhead catenary wires) and any train of any length can "work" because the traction motors in every car are sufficient to push that car with some flexibility (some can push up to two additional 'dead load' cars.) But as others have pointed out, in the US the tracks are primarily straight, the existing traction motors in diesels system works well enough, and there is little incentive to change. That said, one of the options on the table for electrified CalTrain is MU equipped cars so that train sizes can be varied depending on commute load.
  - cbr 12y ago
    MU allows you to have multiple locomotives controlled together, but individual train cars still generally don't have traction motors. There are specialized traincars that have motors but not generators [1] but I don't think these were what foobarian was referring to. [1] http://en.wikipedia.org/wiki/Slug_(railroad) http://en.wikipedia.org/wiki/Slug_(railroad)
  - ubernostrum 12y ago

lutorm 12y ago

The article totally misses the fact that the static friction to overcome on the cars are a static torque not a force. The forces on the cars are acting on the coupling and at the wheel contact point, but the static friction to overcome is at the axle diameter. The torque arm is such that the force there is larger by the ratio of the wheel and axle radii. The larger the train wheels, the less effect the static friction will have.

js2 12y ago

Huh. http://en.wikipedia.org/wiki/Slack_action http://en.wikipedia.org/wiki/Slack_action "Loose coupling is necessary to enable the train to bend around curves and is an aid in starting heavy trains, since the application of the locomotive power to the train operates on each car in the train successively, and the power is thus utilized to start only one car at a time." An example at 2:20 in this video: http://www.youtube.com/watch?v=9D9zE6mju48&t=2m20s http://www.youtube.com/watch?v=9D9zE6mju48&t=2m20s

refurb 12y ago
So how do you get a locomotive started when the engine and cars are not on flat track? If there is any kind of incline, you'll have no slack in the couplings. Can you use the brakes somehow?
- deleted 12y ago
  [deleted]
- tempestn 12y ago
  I'm guessing you answered your own question. If they put the break on the last car, they could let the train compress backwards before starting. Naturally it would still be more work than starting from flat though, so obviously there would be an incline limit beyond which a given train couldn't start regardless.
  - refurb 12y ago
    Can the last car's brakes hold the weight of all the cars in front of it? Can the conductor apply each car's brakes independently? Interesting stuff!
    - tempestn 12y ago
      The original article stated that a train wasn't able to start because the caboose's brake was on, so presumably the answer to the second question is yes, at least for that car. Presumably the first question would depend on the slope of the incline (although if necessary other brakes could be engaged after their cars' couplings were compressed). Anyway, all just conjecture on my part!

femto 12y ago

It seems like cheating that the answer to "How Do YOU Get a Train Moving?" is "With a locomotive." I'd answer "with a crowbar". I used to work in a railway museum and when we didn't have an engine in steam we'd do our shunting with crowbars. The technique is to get a 6 foot long bar with a bent end. Wedge the point of the bent end between the tread and rail, so the "elbow" is resting an inch or two behind the wheel and the bar is up at a 45 degree angle, so forming a lever with a lot of mechanical advantage. Put all your weight on the bar. Eventually the car will start moving. As the car moves, release the bar, slide it forward and repeat. Eventually the car will be moving fast enough that you just run along beside it, sliding the bar forward and jacking up and down with your hands. One person can move a typical car on the flat, but if that's not enough, a person with bar on each wheel will get most things moving. Always have someone on the handbrake, as you will kill yourself if you try to stop the car with the bar!

hueving 12y ago
>and jacking up and down with your hands. Some of the best tools to jack with!
- femto 12y ago
  Had to make it obvious! Once the car is moving, if you try to continue jacking by jumping on the bar you run the risk of going under the wheels. The wheels can easily slice you in half.
samstave 12y ago
Seems like all rail cars need a cyclical version of this, an elbow that can be lowered, and with a circular crank, be wedged and turned to produce forward motion. Or a wind-up where a spinning lever could compound effort onto an axle and drive the car forward...
ohwp 12y ago
Other answer: with a lot of model trains: https://www.youtube.com/watch?v=vC-dmKHiCnY#t=445 https://www.youtube.com/watch?v=vC-dmKHiCnY#t=445

epx 12y ago

I think that having to appeal to couplings' slack to be able to start a train was used in the steam era, because steam engines are constant-force machines (limited by steam pressure and cylinder area). Diesel-electric or electric machines can start stretched trains, even manuals of 1950 engines advise to start stretched. Starting slacked is dangerous because of the impacts on couples (current trains are much, much longer than steam-era trains).

rb2e 12y ago

From my father, who used to drive steam and diesel locomotives in the UK on a Heritage railway (2ft Gauge). He says strictly speaking, legally you're not allowed to reverse a train once passengers are inside without permission, but you can "rock" the locomotive forwards & backwards if you're stuck. Procedures may differ in the USA.

VLM 12y ago
Also the reasoning is you can derail a collection of (relatively) lightweight cars by pushing behind them with a very heavy engine, but its much harder to derail by pulling. Think of trying to keep a rope straight by pulling on it, vs trying to keep a rope straight by pushing on it. The passenger train rule is because its hard to kill people by tipping over coal cars especially when its a rare failure mode anyway. But you wouldn't risk it with cars full of passengers. (Oh and edited to add WRT switches, its subjectively 10 to 100 times harder to derail on a switch going one direction than the other, so again you'd risk backing a bunch of coal cars thru a switch but it would be pretty dumb to do it with cars full of passengers)

symmetricsaurus 12y ago

I am fairly certain that trains do not need the slack between cars to get moving. Like others have pointed out, trains seem to be perfectly capable of starting on inclines where there would be no slack. There is one assumption in the article that is certainly wrong: "...but it seems crazy to think that the train’s friction coefficient is 10 times more than the cars" This does not make any sense. The forces we are comparing is the friction between the rail and the wheels in the case of the engine and the friction in the bearings of the axles in the case of the cars. There is of course some frictional force from the deformation of the track and the wheel as well but as they are both quite rigid (being made of steel) it should not be very significant.

hvm 12y ago
The article has the physics completely wrong. Static friction is what opposes movement between two surfaces that are static next to each other and kinetic friction is what slows down movement between two surfaces. Static friction is always >= kinetic friction. This is explained very well in here (the top picture says it all): http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html Think about how when you start your car from a standstill you need to be careful about your clutch release but when the car moves even the tiniest bit you can even start in the second gear if you want. So, the engine car's wheels get kinetic friction (because they're rolling) and the other cars' wheels (which are stopped) are blocked by static friction. When you have slack and you move one car at a time you only need to beat the static friction of one car at a time. Even if the difference is 10%, if you have 10 cars and it starts to add up. I'm not saying slack is required for starting the train, just that there is an increased difficulty in doing so without slack.
- hvm 12y ago
  Also, this is why doing a wheelspin is never a good idea when you have trouble starting your car/train. The traction when the wheels are blocked but the engine tries to move them is always a lot better than when they're spinning in place.
  - 12y ago