I've had this idea of feeder trains that rendezvous with the primary train by pulling along side, matching speed. Then a brief and orderly movement of passengers between feeder & primary train occurs. When complete, the feeder train disengages, slows back down — the primary train continues to its next destination.

Feeder trains bring new passengers on board the primary train. They leave the station a little ahead of the incoming primary train in order to accelerate up to its speed just at the rendezvous point.

Another empty feeder had already rendezvoused with the primary before the station and passengers disembarking were offloaded onto it.

In short, the feeders can be small. The primary train need not stop at all the points in between.

I would love to model this graphically.