But people want to board the train at any station, not just at the point of departure. If your train is a stack, you can't support arbitrary trips without exces

But people want to board the train at any station, not just at the point of departure. If your train is a stack, you can't support arbitrary trips without excessive delays. Transport optimization is an established subfield of operations research and theoretical computer science. The algorithms for something like sorting railway cars are quite interesting, as they have to deal with physical constraints not present in networks and software.