No mind of it's own...

One comparison of the intelligence of even the fastest & most complicated CPU ever built...is still to the lawnmower...
You can bet, there is an anomaly in your route, dispatching, signal-configuration, or that the task has reached a point that cannot be accomplished(read the above list of anomalies), without taking the throttle(I use trackmarks to avoid this).
There is a bug somewhere, time and again I have found it & it wasn't the game.
It could be a signal-thingy box, or a whistle post, or an invisible signal, that was placed by you or the route-builder, that is actually a signal & fouling-up the ops in the worst of ways. You can see these only with the mini-map, or map-view in Driver.
A junction missing a lever in a yard, a yard with no signals or cars to divert traffic, a change in trackwork that altered a previously trouble-free route, abuse of a directional, or priority marker(I don't use these because of this).
You would need a second monitor to open Windows Task Manager & watch for things like TrainzUtil.exe, that could be called if one of these "anomalies" affects the train in question.
Now, go do your homework!