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Comment by pbhjpbhj | Hacker News Reader

I wonder if you could fit a magnetic braking system mainly at platforms, when engaged trains would be slowed. That could reduce weight on the trains themselves?

Thinking wildly, tube trains are like pistons (they're actual plungers, I guess) if you had a smooth walled section of tunnel then you could have a seal at the head of the train that fans out and provides pneumatic braking, you could maybe use the pressure to drive the ventilation system. Unlikely to be practical (you'd have to have a pre-platform door to prevent all the passengers from getting exposed to the pressure increase, I think).

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Why not just use air pressure for the whole thing? https://en.wikipedia.org/wiki/Beach_Pneumatic_Transit

Magnetic (eddy current) brakes work by converting kinetic energy into heat, which doesn't really solve the problem here. I'm not sure if anyone has devised a system that actually captures electrical energy from the braking action, or if such a thing is even plausible.

Regenerative braking generates electricity and there's really no reason modern trains wouldn't use them.

Another approach that can be used when initially building the system is to have a slope leading into/out of stations so that energy can be stored as gravitional potential. I know the Montreal Metro was built this way.

Your second point was stated exactly in the parent of your parent comment.

I don't think that's correct. @pbhjpbhj talks about magnetic braking and some sort of pneumatic braking. The sibling to his comment by @fredley mentions it, but I don't recall seeing that yet when I posted mine.

Yes. It's called "electric generator" and can be performed by almost all electric motors.

Indeed. Electricity and magnetic fields are so closely related we have a single term, electromagnetism, the motor and generator are thus almost the same thing and it's relatively easy to build a single device which can serve both purposes.

Actually regenerative braking is common on electric trains. Stock bought since at least the 1990s for London Underground has regenerative braking of various forms, with the more modern being fitted to S-stock (the sub-surface lines which were all gradually replaced with a single type of train, if you went on a "Tube" train and it was actually walk-through the length of the train and seemed vaguely modern inside, that was S-stock and you weren't in the deep tube where heat is a bigger problem)

In the 1970s when the oldest stock still in use was purchased, regenerative braking was not a thing.

Regenerative braking was a thing for decades before the 1970s.

Some old freight routes used electric haulage because the regenerative braking going downhill almost completely offset the electricity consumption going uphill.

You could put the braking resistors above ground, no?

Or if the braking resistors have to be close the the coils, then you could use a heat-exchanger to move the heat somewhere else...

It'd at least solve the dust break-pad dust issue

piston effect already does a lot of ventilation work

the more elegant solution in use around in the world is to put the stations on hills

Super pedantry, but I think it's a plunger effect unless you seal around the piston?

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