Ok, here's the last batch of settings I've tried:
Settings part 1
Settings part 2
I've tried 30fps limit as well, I've turned off things like shadows and grass and I'm still getting the sort of frame drops which make it unplayable - I had MSI afterburner running whilst running the game and would often see freezes, or drops to single digit FPS. Yet, the MSI afterburner overlay shows the GPU utlisation staying at 45% or below and CPU cores all capping out at less than 40% utilisation, so it's not even like there's a sudden bottleneck in either the CPU or GPU utilisation.
I am now trying to uninstall and reinstall the game and seeing if that helps, but if anyone has any other suggestions I'd be delighted to hear them. I want to be able to play SimRail, but as it is currently it's utterly unplayable for me. If I can't find a solution, I'll just have to stick to TSC and TSW for my train kick!