My understanding of the substitution feature is that each loco or train car has certain variables set (length, type of power, what it carries, etc.) and that other vehicles that match those variables can sub into the service when this is enabled. Currently each route has a predetermined set of options so that the "right" cars end up running in service mode. This way you don't see CSX engines roaming around Peninsula Corridor or certain German trains running where they don't belong. Some people think this is great. This setup as it currently is now should stay as the default. But it would be nice to have a toggle to allow for anything you own to sub into service mode. I'm not saying we have to see a 182 pulling into Penn Station (although I'm not opposed), but I would at least like to use the Class 33 and 52 on the other British routes even if the purists know this is unrealistic.