If the main route is set correctly using priority markers the AI will run through at an even speed this assumes you have correct signals, track markers etc, if however you have diverging routes and both are " main " this is where you can run into trouble.
First try different signal patterns , ie visible/invisible and none, if this does not work the only sure way is to act as Signalman/Dispatcher and manualy change the point direction before the train arrives it will then go through the complex at the required speed. I find most places have a main route so this second option is not often needed but works well when implemented.
Give it a try it might just be what you need.
Cheers
PJ