Is there anything existing that avoids having to stop the whole train at each stop? For drop-offs, just detach the back train car at a stop. Everyone who wants to get off just moves to the back train car at each stop. For pick-ups, some relatively high-ish speed coupling to add a car at the front of the train. In this way, the train loses one car and gains one car per stop. And as the train progresses, it also slowly turns around, ready for the return trip. Sidings at each stop can allow the boarding and un-boarding process to happen without tying up the track for through-traffic. Seems like something like this must have been tried at some point. Why is this not a thing?
It would remove all flexibility in capacity between different towns, and require most people to move to a different carriage at least once after boarding — which is especially inconvenient with bags. Even if you couple the carriages at high speed, how do you connect the doors between them sufficiently safely to allow people to move along the train?
The coupling of modern high-speed trains is also a significant part of their crash safety. Keeping the train connected if it derails means there's less chance of a loose vehicle punching through another. (See how e.g. [1] has most of the vehicles pretty much intact.)
I think in most cases, running an express train followed by a stopping train is good enough.
