A couple of things come to mind.
Lack of signals. The AI drivers "see" a certain distance and if they see no signals, they will run at 1/2 the line speed.
Running at 3 kph indicates there's a red signal ahead, visible or not, due to a junction (switch) set against the driver. Again, having periodic signals leading up to the junction will help this, and setting the switch for the track the train is running on will ensure that the AI will drive at full speed.