make algorithms have a notion of capacity/exclusion

[golvok]

Can't have more than one train in the same place... and real world restrictions are usually quite restrictive

Also, IRL, there are quite often multiple rails. This can be represented using multiple parallel edges, but may want to "associate" them: the current algorithm will tend to distribute the wanted capacity evenly between them, which may make the calculation more of a "expected congestion if all passengers were to just make their own way" more than where trains need to be.