Annoyingly this actually comes with a light engine formation which I just assumed I would be able to copy and then create new ones from. But I can't seem to find any files for it at all on my system even though the screenshots prove it is there At the moment it will only appear in Free Roam and then only as a light engine.
If ever a loco needed Livery Editor access its the Class 31. 65 years of service on the railways and a multitude of liveries that must be second only to Class 47, it's a travesty that it has been quietly abandoned in game. Great to see someone is fighting for the cause, but sad that it still cannot be let loose to run in-game!
You can also spawn some loose wagons to make a consist? Or take a complete train spawn it, un couple the locomotive, delete it and spawn your reskin.