Welcome to the forums. Do you have an example, which loco do you want to use at which route? Do you use off the rails mode? You really should be far more specific if you want to ask a question or if you want to report a bug. You may read the sticky post on submitting bugs first. For questions, please use the discussion forum.
So I believe to know what you mean. When creating your own scenario and wanting to use other trains than the ones native to the route, you will have to check the "off the rails" option. When you go back to the point where you can click on your custom scenario there should be a key to access the scenario options where you can enable off the rails.
