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> if we run a system for smaller trains, we can build smaller stations for these trains, saving a huge amount on station costs. This costs us in reduced total c
by thesz 1y ago
> if we run a system for smaller trains, we can build smaller stations for these trains, saving a huge amount on station costs. This costs us in reduced total capacity, but this can easily be made up for by increasing train frequency.
There is a safe minimal distance between trains, in fact, a safe distance for a given speed. Shorter trains are not exempt from obeying it. You can make shorter trains more frequent at the expense of lowering traveling speed.
What is the cap of throughput is due to these speed limitations is an exercise left for the author of the article.
- Denvercoder9 1y ago> What is the cap of throughput is due to these speed limitations is an exercise left for the author of the article. They already did that exercise: > 3-car trains running at 30-40 trains per hour (a normal peak frequency for automated or even some human-driven metro lines) reach a capacity of about 18,000 passengers per hour per direction, well above the expected demand of any American line that doesn’t go through Manhattan.
- ghostly_s 1y agoYes, they are assuming a best-case scenario. Driverless systems are very expensive for reasons that have little to do with the cost of the driverless trains, if you're not going to consider those variables this kind of armchair speculation is a waste of everyone's time.
- decimalenough 1y agoThey aren't though? If you're building a new line, fully driverless is pretty much the default these days, especially if the line is fully underground or elevated. What is incredibly expensive, though, is retrofitting a line designed for manual operation to run automatically instead.
- odo1242 1y agoWell, a lot of systems exist that were initially designed for automatic operation but still end up becoming operated manually or partially manually due to safety concerns or politics. Washington DC Metro and BART are the two big systems I can think of that had this issue.
- theluketaylor 1y agoBoth examples of Great Society metros that were on the bleeding edge of what was possible in the early 70s. Automatic train control advanced rapidly, with both Vancouver SkyTrain and London Docklands Light Rail being built in the 80s and operating driverless for their entire existence. DC Metro just recently re-enabled full automatic train operation across all the lines in June.
- odo1242 1y agoI see
- decimalenough 1y ago40 trains per hour is in fact not "normal", but extremely difficult. Only a few systems in the entire world operate more than 30 per hour. The fundamental constraint is not technology, but people and physics: you need to decelerate and stop, let people disembark and get on, accelerate and clear the platform. This cycle requires a bare minimum of 90 seconds, although IIRC a few lines in a few places like Paris and Moscow do 85 secs.
- matt-p 1y agoIndeed, the Victoria line in London manages 36 TPH and we've not bothered beating it since. It's much easier to run 26-30TPH with slightly more carriages.
- lmm 1y ago> the Victoria line in London manages 36 TPH and we've not bothered beating it since That was a world record for a line following modern safety standards, set less than 10 years ago. It's hardly a case of "not bothered", it's just hard.
- jefftk 1y agoSEPTA's T [1] gets up to 70 TPH and used to handle 150 TPH. You can do this with multiple trolleys loading/unloading on a platform simultaneously. (But this strategy is orthogonal to the article, because it requires long platforms.) [1] https://en.wikipedia.org/wiki/T_(SEPTA_Metro) https://en.wikipedia.org/wiki/T_(SEPTA_Metro)
- bobthepanda 1y ago90 seconds is very possible in new-build lines which is what the author is talking about. You can buy a turnkey Innovia (e.g. Vancouver Skytrain) or AnsaldoBreda (e.g Copenhagen) that does this out of the box. Retrofitting 90s operation is basically impossible but not the point of this exercise.
- thesz 1y ago> They already did that exercise No, they didn't. They took "30-40 trains per hour" out of thin air and exercise was to calculate whether it is even possible to have more frequent shorter trains.
- clickety_clack 1y agoFor capacity calculations, headway is what matters. E.g. trains spaced 2 mins apart means that 30 trains run in an hour. It’s the same with cars. A 2s headway with cars holding 1 person each means that the maximum capacity of a highway lane is 1,800 people per hour, no matter how fast they go (the cars are further apart at higher speeds).
- Animats 1y agoFreeway capacity is maximized around 35 MPH. Faster, and the greater distance between cars reduces capacity. Slower, and there are not enough cars per minute per lane. So the goal of ramp metering signals is to throttle input to keep the freeway speed around 35 MPH.
- mike-the-mikado 1y agoInteresting - I have believed for many years that it was around 17 MPH. I felt that this tallied with my observations - as traffic levels increase, vehicles slow down (increasing total capacity) until it falls to a critical speed (when slowing down reduces capacity) and then it changes to stop/go. In my experience (on UK roads) this critical speed is around 17 MPH - but it might be a little different elsewhere.
- namibj 1y agoFreeway capacity chokes at the limit giving rise to basically overpacked lanes slumping from metastability; systemic control with ramp meters or I66 (within the Washington D.C. Metro area) style real-time dynamic pricing lets freeways flow properly around peak capacity if properly implemented. There are roads that regularly suffer from acutely insufficient capacity in many metro areas; specifically, repeatedly at times _the dynamic pricing toll that would discourage enough people from using it to stay uncongested_ would overshadow the price of a rental-with-driver (Uber-style) during off-peak times. It's not that the people shouldn't get through; it's that most people won't need more than a backpack worth of luggage with them and could thus be packed 3~4 passengers for each driver. Splitting the toll would be the reason to do so. Unfortunately only really dynamic congestion tolls would really fix the concept of rush hour traffic jams. And the necessary surveillance system would bring severe mass surveillance/tracking concerns with it at least in central Europe.
- markasoftware 1y agoI wonder if it's possible to run trains at higher speeds closer to each other using fixed brakes embedded near the tracks, similarly to how roller coasters often have mid-course brake runs that are only activated in emergencies when the train ahead unexpectedly slows or stops.
- Symbiote 1y agoRoller coasters accelerate and brake much harder, which is fun for the restrained passengers but not for commuters. Signaling systems used on automated trains know the position, speed and capabilities of every train. Keeping a safe distance behind isn't a problem.
- namibj 1y agoWe're at the point where we could easily fit so large a fraction of the rail length covered/overshadowed by the train with https://en.wikipedia.org/wiki/Track_brake https://en.wikipedia.org/wiki/Track_brake that we could pull around 4 G deceleration if we cover the bottom in that brake chains (chain strong; chain flexy enough to just wiggle over humps in the track).
- bobthepanda 1y agoWhat works for a strapped in amusement park rider doesn’t work for the standing commuter holding a cup of coffee. Trains have the capability to accelerate and decelerate faster, we mostly don’t do so for comfort and safety reasons.
- rangestransform 1y agoI don’t think anybody has tapped into the forbidden magic trick of having CBTC broadcast position and velocity instead of position alone. For vehicle ACC at least, there is a safe region of velocity and follow distance where a following train can plan to enter the space where the lead train currently occupies.