5 ms·

i wonder about the brake dust in not only cars but trains as well. maybe where trains don't decelerate much there is less pollution?

liability 6y ago
I'm not sure, but I think modern trains usually use electrical braking instead of friction.
- deleted 6y ago
  [deleted]
- rootusrootus 6y ago
  Perhaps an all-electric train could, do they just feed it back into the grid? Freight trains mostly use air brakes, which are friction.
  - liability 6y ago
    A diesel-electric freight train locomotive can use dynamic braking by dumping the electrical energy into a bank of resistors (known as rheostatic braking, opposed to regenerative braking, but collectively called dynamic braking.) However apparently this form of braking is only a small portion of braking that freight trains do since only driven axels can participate in it and most freight cars are unpowered. I didn't consider that.
- detaro 6y ago
  As far as I know only rarely, and even those trains do have mechanical brakes too. Modern electric locomotives can do engine braking, but brake power needs to be applied everywhere on the train, so most of the train brakes mechanically. (And if you want to engine brake, the energy still needs to go somewhere - so you have additional complexity of feeding it back into the grid or you are limited by how much energy you can dump to heat) Only (mostly high-speed) passenger trains with driven wheels everywhere can brake well only electrically for non-emergencies. Some also have additional eddy-current brakes that interact with the rails. For the German high-speed trains they try to design the speed profiles so that they have to waste as little energy as possible by using the motors as generators and feeding it back to the grid. But they still have and use eddy-current and mechanical brakes too. (You can tell the difference if they are delayed: they brake later and more aggressively to make up time. And a full-on emergency stop is loud and smelly.)
  - ehnto 6y ago
    This was part of the design behind the N700 bullet trains of Japan. They have motors in all trains in order to distribute weight and thus braking and acceleration ability evenly across the whole train. It reduces wear on all wheels and components but as you mentioned adds complexity in the overall aparatus. They brake at stations using mechanical brakes though if I am not mistaken, the sound is pretty hard to ignore!
MarcelGerber 6y ago
At least for freight trains and normal passenger trains (especially diesel), that's probably true. High speed trains usually use induction brakes (regenerative braking) and as such, they don't produce nearly as much brake dust. Also, freight trains generally don't need to brake very often, since the tracks are free for kilometers to come most of the time. In general, trains are of course far from emission-free, but as they can transport way more passengers/freight the emissions per person / tonne of freight are still lower than cars or trucks.
- ehnto 6y ago
  Electrified rail also gives the promise of eventually being low emissions when we switch to greener power plant. So it benefits from scale economics twice right now, and will seamlessly switch to entirely green when the power plants do.
  - MarcelGerber 6y ago
    That's true. In Germany, Deutsche Bahn boasts that its electric long-distance trains are 100% powered by renewable energy. There are some asterisks to that, but it's still a great thing. So yes, electrifying lines really has many advantages: * it allows for high-speed trains * it enables trains equipped with regenerative braking * its trains are driving more efficiently (most diesel locomotives actually are diesel-electric, meaning they have a diesel generator powering an electric drivetrain) * it allows the trains to be powered by renewable energy All these points drive the overall carbon footprint down in one way or another.
    - saagarjha 6y ago
      Plus I would think that they're quieter?
      - rsynnott 6y ago
        MUCH quieter. I occasionally use a diesel commuter train; shockingly noisy compared to the usual electrics.
scythe 6y ago
Friction braking of a steel wheel on a steel rail produces fewer particles (more ductile) which have a higher density (iron > carbon), so the accumulated air pollution should be much less.

liability 6y ago

I'm not sure, but I think modern trains usually use electrical braking instead of friction.

deleted 6y ago
[deleted]
rootusrootus 6y ago
Perhaps an all-electric train could, do they just feed it back into the grid? Freight trains mostly use air brakes, which are friction.
- liability 6y ago
  A diesel-electric freight train locomotive can use dynamic braking by dumping the electrical energy into a bank of resistors (known as rheostatic braking, opposed to regenerative braking, but collectively called dynamic braking.) However apparently this form of braking is only a small portion of braking that freight trains do since only driven axels can participate in it and most freight cars are unpowered. I didn't consider that.
detaro 6y ago
As far as I know only rarely, and even those trains do have mechanical brakes too. Modern electric locomotives can do engine braking, but brake power needs to be applied everywhere on the train, so most of the train brakes mechanically. (And if you want to engine brake, the energy still needs to go somewhere - so you have additional complexity of feeding it back into the grid or you are limited by how much energy you can dump to heat) Only (mostly high-speed) passenger trains with driven wheels everywhere can brake well only electrically for non-emergencies. Some also have additional eddy-current brakes that interact with the rails. For the German high-speed trains they try to design the speed profiles so that they have to waste as little energy as possible by using the motors as generators and feeding it back to the grid. But they still have and use eddy-current and mechanical brakes too. (You can tell the difference if they are delayed: they brake later and more aggressively to make up time. And a full-on emergency stop is loud and smelly.)
- ehnto 6y ago
  This was part of the design behind the N700 bullet trains of Japan. They have motors in all trains in order to distribute weight and thus braking and acceleration ability evenly across the whole train. It reduces wear on all wheels and components but as you mentioned adds complexity in the overall aparatus. They brake at stations using mechanical brakes though if I am not mistaken, the sound is pretty hard to ignore!

MarcelGerber 6y ago

At least for freight trains and normal passenger trains (especially diesel), that's probably true. High speed trains usually use induction brakes (regenerative braking) and as such, they don't produce nearly as much brake dust. Also, freight trains generally don't need to brake very often, since the tracks are free for kilometers to come most of the time. In general, trains are of course far from emission-free, but as they can transport way more passengers/freight the emissions per person / tonne of freight are still lower than cars or trucks.

ehnto 6y ago
Electrified rail also gives the promise of eventually being low emissions when we switch to greener power plant. So it benefits from scale economics twice right now, and will seamlessly switch to entirely green when the power plants do.
- MarcelGerber 6y ago
  That's true. In Germany, Deutsche Bahn boasts that its electric long-distance trains are 100% powered by renewable energy. There are some asterisks to that, but it's still a great thing. So yes, electrifying lines really has many advantages: * it allows for high-speed trains * it enables trains equipped with regenerative braking * its trains are driving more efficiently (most diesel locomotives actually are diesel-electric, meaning they have a diesel generator powering an electric drivetrain) * it allows the trains to be powered by renewable energy All these points drive the overall carbon footprint down in one way or another.
  - saagarjha 6y ago
    Plus I would think that they're quieter?
    - rsynnott 6y ago
      MUCH quieter. I occasionally use a diesel commuter train; shockingly noisy compared to the usual electrics.

scythe 6y ago

Friction braking of a steel wheel on a steel rail produces fewer particles (more ductile) which have a higher density (iron > carbon), so the accumulated air pollution should be much less.