The hardest part of this game is getting the AI drivers to do what you want them to. If there is a way to avoid doing what they are supposed to they will find it. There are a couple of tests you can do first. Drive a test loco over the affected area yourself and if it derails you have a problem, lol. Also check all points to see if they have a red and green arrow. You may need to install or replace a point motor/ lever. The second test is to place a series of track markers at intervals along the route called "test 1, test 2 etc!. Then instruct your test loco to drive via the furthest one. Let's say it is "test 6". If your Ai driver won't go try "test 5" and so on. That will identify the affected area of your route. You may find that you need an additional red tracker so you can add a "via" command to make your driver behave. You may also want to consider placing a direction marker to stop you naughty driver going into the siding.