I know with 04, and 06, while the AI might have some logic problems when picking the best route, (its not that big of a deal when it comes to trouble shooting, you can make a really busy, complex route train wise as long as your computer can handle it. Portals will turn trains around and also spawn new ones. If you want you can have a train a minute running spawning out of each portal(though i dont recommend it. It will realisticly create a major traffic jam.)