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Comment by thingie | Hacker News Reader

Hardly an innovation. Various people already had the same idea so many times. It seems quite an obvious and tempting thing to do.

http://blog.modernmechanix.com/2008/12/18/train-picks-up-and...

Well. It's pretty much unworkable. Train is not a single rigid body, it's composed from bogies and car bodies, and they are only quite loosely attached. They absolutely have to be. So you can't expect it to just sit as a rock on the tracks, even in pretty low speeds. And with speed low enough, you are maybe better to simply stop. Modern electric bullet train unit with high powered axles ratio can accelerate really, really fast.

Anyway, you could spend money much better on allowing higher speeds on the whole track, or improving adjacent tracks so to allow higher speeds even for feeders and conventional trains, which could allow you to make fewer stops.

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Modern electric bullet train unit with high powered axles ratio can accelerate really, really fast.

Yes, but accelerating the entire train is going to cost way more energy than accelerating the transfer car. Also, the transfer car can simply use wheels and brakes to gain kinetic energy from the train, so it can be incredibly light. Even if a longer track and motor are necessary for high speed bullet trains, there would still be a big energy savings.

Also, no matter how quickly you can accelerate to make a stop quickly, the same train will be able to save considerable trip time with the transfer car system. Saving a half hour or hour off of trip time from a route is certainly worthwhile, especially if it means that the train can run the same route at a lower top speed. The energy savings from that could be enormous over the long term.

I'd also thought of this independently in the past, presumably lots of people have. As for decelerating, I have no idea if any modern trains do this, but you could in theory charge a battery with the kinetic energy from decelerating the train and use that for acceleration. Naturally it wouldn't be a perfect energy transfer, but I presume much of it could be captured.

It's quite common. Except that there are no batteries (battery won't do, you can't charge it fast enough), you just need to have another train in the same section to consume it (or whole network, if your converter substations are ready for that). Energy savings estimates were like 10 to 20 percent (don't have any link for this, it was some British study I remember, sorry).

http://en.wikipedia.org/wiki/Regenerative_brake

Seems like it's everywhere these days. Even in urban metro lines where you've got like 1km between stations. HK subway trains have been doing this since the 90s. Wikipedia sez Delhi Metro has it too.

It's most useful in such cases (and trams and so on), with very frequent braking and starts and a lot of trains. There is always some other train accelerating. And you have majority (if not all) axles powered, so you can have regenerative brake on all of them, allowing you to brake effectively only with the regenerative brake to almost zero. It's even much more silent than a mechanical brake, also an advantage in urban environment :-)

(And even if you don't give the energy back to the network and just burn it in the resistors in locomotive, it gives you more effective braking. Locomotives from 1960's already had that.)

Well, even just weight of the people in the transfer car will make it quite heavy (few tons at least). And you are not going to make this for just 10 people at each (non)stop.

You could save some energy by using only recuperating electrodynamic brake to zero to stop the train and store it in some supercapacitators (not on the train, of course) if there isn't any other train to use it. Because you have many powered axles, you could brake quite effectively and fast enough. Well, possibly still not fast enough to use just EDB.

Raising top speed is of course going to cause much higher aerodynamic drag, so if the energy use is primary concern, it's not exactly the right thing to do. I rather meant eliminating any drops in the average traveling speed except the stations.

But I can't really argue against energy savings, because most likely, there would be energy savings :-) If you could built such a system safe enough (I don't believe that) and with less than absolutely insane costs (I don't believe that either :-)).

The comments on that link point to an interesting poor man's variant (disembarkation only): http://en.wikipedia.org/wiki/Slip_coach

Pretty obvious I suppose, but interesting that it was actually used in routine service for a few decades.

I've thought inmediately of Asimov's The Caves of Steel where pedestrians jump from a moving band to another, some slow, some faster.

For trains it could be nice to use small wagons that could change from a train to another, not because of energy saving, but to achieve more speed, avoiding the time needed to change lines. I agree with a previous commenter that trusting passengers to behave correctly is looking for problems.

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