Depends on the software. You mentioned RailClone, so I assume you work with 3dsmax? There are a lot of articles and blogs about parametric modeling in 3dsmax, but most of them are based on using just the modifier stack with editable poly. There are also many out there for Para3d. That's probably one of the most popular "true" parametric modeling plugins for 3dsmax. Plenty of videos on Para3d out there too.