When I have an AI running problem I tend to choose Easy Mode to sort it in. When a train will not move due to an apparent tack issue I go to the Driver Commands menu for that train along the bottom of the screen and click on the left most arrow icon just before the first Driver Command the train is not obeying to bring up the Command Menu. I then select Stop Train which in turn brings up the Basic Loco Controls (bottom right hand corner of the screen). Use the Basic Loco Controls to set the train in motion at a medium speed I then go back to the Command Menu and select Continue Schedule which it will fail to do due to current track issue.
I then follow the train until it either fall off the end of a broken track or AI again takes control and the Basic Loco Controls panel disappears. Either way you have located the issue. Broken track gets fixed and an intended track end may require a buffer or signal. At points, other than ensuring the track is actually connected, delete the lever and replace it, its quicker and more accurate than trying to relocate it. In the middle of nowhere its frequently a rogue points lever left behind after a route change which could be miles away and sometimes a reversed signal, wrong side of the track like two way working.
I've known trains which will just not follow close to another, change their running order or separate time wise and it all works, unexplained a Trainz quirk. I sometimes get the impression laying single track in the direction of travel tends to work better but accept that could equally be a quirk of me. Peter