I don't think you understand AI AI applies to any train not driven by the Player. When you drive down a route the trains going past or in sidings are other trains you own. You can add repaints of trains you own or buy new trains you don't own. It is the Scenario you are driving which decides what type of trains you will see. In a Quick Drive Scenario the other trains can be random In a Standard or Career Scenario the other trains are fixed in the scenario - every time you play it you will see the same trains at the same locations.
Yes. Use the scenario editor functionality. Details in the manuals that came with TS. There are lots of tutorials on YouTube.
And if somebody had said that to you a short time after getting the game would you easily work out the scenario editor?