Also - is there some special (sub)forum for feature requests? Was it already suggested in some shape or form? Am I missing some way to accomplish this already?
Yes, there is, moved to Ideas and Suggestions.

Several Ideas around train routes / pathfinder have already been collected here, where I ...

So I'm building a(n overly complicated, for sure, ) train network and one thing that I find missing is a way to direct trains to particular routes.

The reasoning is simple: with many paths between several stations, path finder usually uses the shortest one - which may be the most congested, or ...