This whole sub-forum is a great surprise to me, I am not the only one who does these type of models
My project was based on a H0 Piko train set, as H0 system is the cheapest, easily available and most widespread system of them all. I simply built new superstructure onto the waggons, keeping their running gear and connectors intact. I was not aiming for one particular type, I tried to keep it generic era-agnostic and fit for both my 28mm sci-fi and 1/35 historical projects, this was possible by using two separate locomotives for the two scales. The trick is that the locomotive is not the locomotive, the bigger, closed car is the real locomotive from technical point of view. This way it is a completely working H0 train model able to run on any standard H0 rail system.
I plan to make bigger locomotives and bigger cars, we will see.
1/35 loco:
28mm W40k loco:
The train with the working locomotive: