The re-topo for the train was a success, and the UVing was torture, but at least texturing can begin now. I took it into Quixel and began playing with metal overlaid with painted metal, overlaid again with rust. I then played with blend modes and colours/ metallic values until it looked good, and began adding the details such as text.
This is the final asset, rendered in Toolbag. There are lots of little details and nuances which really help to make this seem abandoned and part of a decaying area in which it is destined to stay forever. Deep.
Here is the texture breakdown, no metallic as it was just black so I can use a 0 constant in Unreal engine materials to save file size.