the game would not need 4 aspect signals in order to allow for switching of tracks. its all in the pathfinding logic, the signal has nothing to do with it. if the pathfinding logic would allow for choosing of alternative paths on the fly, it would work even without any signals at all.
the signals in train fever do four things:
1. they divide the track into blocks to allow for more traffic on the lines
2. they determine the place where trains stop if the track or path ahead is blocked
3. they define whether a track section can be used in both directions or only in one direction
4. they suggest which side of the track trains should run, even if they are not set to one-way operation