Either each car needs a driver or you need something to keep all the trailers precisely lined up. One new system has that; a human drives the "locomotive" and the trailers have automated steering to follow the lead vehicle precisely enough to not bump into traffic in the other direction. This is still experimental.

At the other end of the line, outside the tunnel, there's a whole rail yard. The different kinds of cars have to be sorted out, loaded, and unloaded. So there's a need for a big staging area.

Here's an overview of traditional tracked systems.[1] I've tried to find a video of the back end system and yard, but nobody seems to make such videos. It's too boring.

[1] https://www.robbinstbm.com/products/tunnel-boring-machines/b...

[2]