Sunday, July 5, 2009

Dismounting at speed

An interesting idea that I've seen several people write up about is the idea of a non-stop train that one would leave by getting onto another train that would stop. The stopping train would then either pull into a station immediately, or cruise about town to lower-traffic destinations.
The primary advantage of this is that the main-line train does not have to stop, ever. This saves energy and runs faster. People going to destinations further down the line are not slowed by people getting off on the station where the train is now, and the train can be refuled and repaired at the end of the line.
The primary disadvantages are space and safety. If someone should be a little too slow in disembarking, suddenly the stopping train pulls away, dropping the slow passenger onto the tracks at speed. Ouch. Space-wise, this means all train stations need to have an extra set of tracks that moves next and paralell to the main trains, for a fairly long distance. (However long it takes to disembark the train, reembark the new passengers, plus two extra minutes for safety's sake all times speed to produce a distance.) This distance can be reduced if the main train slows down, but that lessens the advantages given above. (Only a little bit.)
This probably won't happen for safety's sake. That one slow person getting dropped on the tracks would probably die, and most societies aren't willing to kill for efficiency's sake. (Also, I'm pretty sure at least one person would fall out per trip. Some people are very clumsy.)


l33t MD* said...

Very interesting!

I agree regarding safety concerns, it's probably too vulnerable to transport humans. It could work well for cargo trains, though. Containers are easy to shift back and forth and it's not a complete disaster if one falls off.

One thing you didn't mention (but probably thought of): The auxiliary train must be accelerated to match the main line train's speed and then decelerated back to zero speed. This means that in order to get the energy saving benefit the auxilary train needs to be smaller than the main train (ie. need less energy to accelerate/decelerate than the main train). Optimally the auxiliary train should be just large enough to contain the embarking and disembarking volume of cargo. A computer system would solve this logistical conundrum easily.

I only see this feasible for long-haul, high volume trains though, as you would need many stops in order to get a substantial gain.

Totally awesome idea! :)

themadengineer said...

I am assuming that the auxilary train is smaller than the main-line train -- we're not unloading all the passengers or cargo on any one stop. If we are, we might as well stop the main train.

Anonymous said...

Remember the haunted mansion in Disneyland? moving discs articulate with a train to allow passengers to walk from a stationary platform to a (slowly) moving train.

themadengineer said...

That and similar attractions at other amusement parks show that +/- 5kph differences in velocities should work fine.

(And it's only the difference that would matter, special relativity shows that you can take arbitrary frames of reference for this kind of problem. So we take the frame of reference that the main train isn't moving, the ground is going 140 kph, and the unloading train is going maybe 2 kph forwards, which we'll slow to 2kph backwards when it's almost time to separate.)

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