I want to be able to mix up train cars and engines (with respective countries) to allow for a more diverse range of scenarios to make with these constants.
I presume by constant you mean consist? Well yes it's a good idea and has probably been put forward several times already, along with a decent scenario or timetable editor to do our own train plan. I would say it's more likely to come than a route editor, really depends what the associated UE4 requirements (baking, etc.) are and whether a coherent front end can be put on that.
Also dtg would have to make a clear understanding on how to use simugraph etc to simulate the trains braking axles etc as that's not going to be simple