Remember that the scenarios are usually planned for up to 2-2.5 hours - after that, no trains will be in service and so the routes will be completely barren without any AI traffic at all.
If a train has already arrived and has been diverted, can I put that train back into service or not?
You can, but you must use a renderer which supports shaders. Press Tab - a minimap will open. You can control the signals and junctions from there, which should allow you to freely roam around the scenery. You will not get a new timetable though - the train will be empty and the destination signs will not display anything.
Ah okay, that's quite long.
What is a renderer?
Could I also, for example, change an existing train timetable?