I had the same problem. My routes were fine, until I set up a route in a new city. It seemed like a very simple route. More or less, just a circuit. But the AI turned it into a nightmarish pretzel – twisting & turning, and doubling back and forth.
I sat there, scratching my head – and tried to tweak it, which just made it worse (if that was possible). So I deleted all the stations and started from scratch. Exactly the same result.
After swearing at the screen for 5 minutes, I was just about ready to give up on the town, when I noticed something……….
Most of my stations were on one side of the street, but a couple were on the other side.
After placing all the stations on one side, I had the elegant circuit, I was aiming for. The other side can wait until there is more traffic-flow.
Hope this helps……….