Same problem here (only with Max 2019). I can open the original file (saved before updating to Railclone 4) but if I save it and reopen it, it crashes (Application Error. An error has occurred and the application will now close). Also, when starting a new scene, I am able to merge all objects from the file, but as soon as I try to merge one of the RailClone object the new file crashes as well.
My workaround for now is open a new file, merge everything except the Railclone objects and rebuild the Railclone objects...