I think CSG is solid modelling (Unions and negates, that stuff). While I don’t have much experience with chassis systems, maybe you could go more into depth about the problems the new physics system is giving you? Oftentimes in my (limited) experience physics issues can be fixed in advanced game settings or minor script changes can resolve the issue. What exactly happens to the trains when you boot the game? Is it as simple as they don’t function at all?
I don’t really think I’m experienced enough to be worth arranging a full consultation. My only real recommendation would be to experiment with the physics settings in Studio to see if you could make something work more efficiently.
Definetely do this if my recommendations fall flat.