SImplifying the rail's helix into line segments ended up being the only method I managed to apply correctly. That brought the PolyCount down by 1.000.000 polygons. Converting to morph and try to simplify its geometry didn't work out for me because none of the morph edit tools had any effect. However, I confess I am not very skilled using the morph tool so I might have overlooked some functionality.
Anyway, thank you for the help so far. The overall cleanup brought my project down by 1.5million polygons so now, at 2 million on the overall project I can work (somewhat) smoothly again. Probably it's time to get a better machine. Cheers