Trains that decouple and recouple
I've often wondered why, when you have an electric train line that has a terminus as the main destination, you can't give everybody an express train.
To do this, imagine for the commute home, a 5 car train starts downtown. It leaves and expresses a few stops down the line. (A local car leaves after to handle the stops close to downtown.) When it gets to point one, with sufficient warnings and many safeguards, it decouples, and the rear car brakes to stop at the first of its stations.
Passengers get off (and on as well, see below) and the car, which has its own power coupler, takes off to drop folks off at the next few stations. The main train releases another car after that which handles the next few stations.
This has been thought of before, but next the hard part, something needing more modern technology. After the drop-off car has completed its local run, it would attempt to rejoin the next express train, allowing local passengers who got on it to get on an express, then move to the car that will eventually decouple to go to their stop. With the right timing this could go on all day.
Not that this is easy...A live docking is a complex thing. Today we might be able to pull it off by combining two different computerized systems with failsafes, and a human overseer. The train cars would have to have giant springs on their couplers to allow the impact to be absorbed. The motors and brakes of both units would need to be in fully computerized sync, and any failure would cause the rear to brake and the front to accelerate (if possible) to avoid collision, though it should be possible to deal with a moderate collision with the springs.
Of course you need special doors at front and back of the cars to allow passengers to flow, big enough for a wheelchair to get through which makes it more complex.
Of course, if all you want is a one-way trip with almost everybody leaving the downtown station and getting off at their own statiions you can do this much more simply, without any coupling or decoupling. Just have N independent cars, and the first car to leave expresses towards the last stations on the line, the next car to the ones before that and so on. The cars can be spaced a fat distance. I presume they don't do this because this travel pattern isn't that common.
Another alternative is to have the express simply stop every N stations and join with the local car that left the last express while stopped at the station. If scheduling finds it easier, a car could be picked up by the main train in either direction, though passengers picked up by the local car would need to get off to get on the next express when it comes through.
This does require that an express go through fairly often. If doing the local hops takes the car 10 extra minutes, an express could run every 15 minutes or even every 20, since the 10 minute wait would be justified by being able to catch a nice fast express.