Trains that do "never" have to stop

Question

What are the limitations of trains that never stop. Can we have shuttles decelerating from the train's speed to 0km/h at the station for alighting passengers, while another shuttle picks up boarding passengers from the station, and accelerates to the train's speed to catch up? (Even if this is only done for a few stops on the whole line, and the main train still has to decelerate slightly, I imagine it will still lead to a fair bit of time saved, especially for metro.)

This seems to be a 'vernacular' idea in the minds of some. I was reminded of it when reading a vague pitch by Peng Yu-lun. There must be many reasons this hasn't worked/won't work that I am missing.

I'm personally most curious about this in the context of urban metros, though it could be relevant to longer-distance rail and HSRs too.

(First time asking a question here, hope it's not off-topic/too unspecific 😅. Personally I wonder about this whenever I'm taking the subway. I'm from Singapore if you'd like to provide additional context.)

Edit: sorry for my slow updates. Thanks for the comments. I never managed to find the phrase 'slip coach' when googling, that helps a lot. Transistor's answer/other answers mentioning safety (if there's a crash) and infra/cost (platform) have been enlightening. I promise to read up and mark/accept an answer eventually/soon.

Answer 1

I worked several decades ago as a railway signal engineer. We used to say that everyone would like an express train that ran non-stop from their origin to destination but, of course, that doesn't suit everybody else on board. Your question seems to try to address the problem.

Using self-powered multiple units coupled together would offer a theoretical solution on normal rails.

Trains move from left to right.

1   [d][c][b][a] 
  Approach             [stn]

2       [d]    [c][b][a] 
  Decouple             [stn]

3                       [d]           [c][b][a] 
  Stop                 [stn]

4   [h][g][f][e]          [d] 
  Next train           [stn] d starts off

5       [h]    [g][f][e]        [d] 
  Transition           [stn]

6                       [h]           [g][f][e][d] 
  Recoupled            [stn]     e, f & g couple up with d.

How it works:

• All passengers for the next station move to the last car or set of cars. (This requires connecting corridors.)
• Corridor doors are closed.
• At normal braking distance from the station the last set is decoupled. The front section runs on at line speed. The decoupled set brakes to a halt at the station.
• Passengers disembark and the set sits there (blocking the line for other traffic) until the next express is due.
• The set accelerates out of the station on time so that it will be at express speed when the express reaches its rear. The express couples up, the passengers don't feel a thing and the journey continues.
• Meanwhile the last set of the express comes to a halt at the station and the procedure repeats.

There are multiple problems with this scheme.

• Any delay or mishap with the shuttle acceleration may result in an emergency stop of the express or a fatal collision.
• Every station along the express route will have a train set sitting idle for most of the time.
• Every station along the route will have to have platforms on the express line. This platform will be blocked for most of the time.

There will be many more problems. I don't think it's a runner.

Answer 2

The limitation is cost. Mechanisms, land for the larger stations, tracks, rights of way, maintenance, etc.

If it paid out to run fewer trains by not having them stop it would already be done. There are edge cases where things aren't done because of social inertia, not cost, but this isn't one. It isn't hard to conceptualize how complicated this would be.

The other problem with a train the never stops is what does the crew do? Just live in multiple places?