I found that if you have many AI trains running at the same time, some of them are prone to make a "mistake" while others run fine. Then, when you save the session, let's say that one consist is at one end of the route waiting for a green signal, (or in a wait for command) and you of course don't know because you are operating at the other end, and save... when you return opening where you left, that train waiting may not continue when the signal goes green (or the command is frozen), sitting there forever. Or in another stance, a train involved in complex AI switching, caught in the save, may run crazy when returning to the saved point. If you know all this, being careful, you may get by with little problems.