An idea that just came to mind: What if, in addition to the locos you actually can drive on the route you selected, also show the ones you COULD drive, if only you happened to own their respective DLC? Maybe show their picture greyed out, pale, black and white, something like that. Clicking it would lead you to the steam/Dovetail store. You could even add some highlighting when they are on sale. It would be especially wise on this one though, to give players the option to turn it off, not everyone wants to see what are essentially advertisements in there. But then, I've often wondered whether this particular loco will run on this particular route - and this way would leave very little uncertainty as to what goes and what doesn't.