In theory it could work. I think one big problem would be the erosion of the rails, slowly drifting apart.

The problem would be the scheduling. Even at the best of times railways rarely run on the kind of precision that these kinds of operations would demand. Plus it’d require a lot of duplicate railway since presumably it would take distance for shuttles to accelerate and decelerate, and the shuttles would need to get back to the original stations in time for the next trains.

This one?

https://www.theatlantic.com/china/archive/2013/12/this-propo...

On most rail routes in Austria we have high speed trains and low speed trains on the same routes. When you need to go to a smaller station, you get off at the closest major station and switch to the slow train. It would be awesome if you could just switch directly to the low speed train whenever the high speed train passed it... (Then you don't even need to have shuttles going back and forth)

This would be awesome.

Seems extremely cost prohibitive. In addition to the extra rails and control systems, you’d need a high speed locomotive and several cars for every single station along the route.