Having many trains on the same network of tracks requires a rail system to be tuned to perfection. If they aren't designed with great care, ratings will sink and money will be wasted by trains getting 'lost' on their way to stations - creating running costs without collecting any profit.

    When the AI is servicing stations that are a long way apart it will have two-track stations at each end, and "passing loops" along the track.

    The first problem with "passing loops" is that each train has to wait for its turn to use the "passing loop" - consequently, the number of days in transit will go up resulting in reduced profit. The second problem is that only three trains can be successfully run on a "passing loop" setup. Why? Because any more than that will result in a deadlock.

    This causes a huge problem - it means you have to continuously watch over your railways to make sure this doesn't occur. And fix it if it does.

    Early on in the game, however, you are probably going to use these silly "passing loops" because of the tight budget (and you can get away with it more easily as you probably won't be able to afford many trains). That's fine, as long as you change your system(s) over to dual tracks

1. as soon as you can afford to
2. as soon as the traffic on that particular line warrants it

    So, what are dual tracks? Exactly that, two (parallel) tracks. But what is significant about these tracks is that one track goes one way and the other goes the opposite direction - just like a road. Here is an example of a dual track system using "one-way" signals.

    On one of the tracks, you need to put a "one-way" signal that faces the direction you want your trains to travel, on the other track, put a "one-way" signal facing the opposite direction. (Being Australian, I have the trains driving on the left.) If you build lots of signals all the way along the track, the amount of trains you can run is enormous, as many as one train for every 6 sections of track!