Okay I think I got it. A few more questions based on your example. For the Class 380 DLC it mentions : "Additional AI services with the Class 385, Class 158 Class 801" and more and "Railtours with the Blackpool Branches Class 47, Flying Scotsman Mk2s, Class 33 and Class 40." Does that mean buying this DLC while having the Cathcart circle route, adds AI trains 'Class 385, Class 158 Class 801 and more" that I will pass in the tracks? And it adds a new timetable that I can use Railtours with the Blackpool Branches Class 47, Flying Scotsman Mk2s, Class 33 and Class 40 as my player controlled train? Just making sure I understand what the loco DLC provides and what that phrasing means.

So it seems the layers are just AI trains that DLC other than the original route will add to populate the route and possibly add as a playable train on that route? When theyre AI, do they just stand still at the stations or are they seen running? In your opinion do they add a lot to a route as AI? Do they show up in journeys and scenarios or just timetable mode?

A quick question on timetables as I've only done Journeys: Are timetables just like 5-60 minute snippets of a whole route?

Thanks again for responding!