After some tests I've decided that this method is simply too complicated and time-consuming. I have animated several RailClone objects with multiple generators each (which I'd have to separate), and I've randomized just about everything (transforms, MatIDs, UVWs), so I'd have to manually create an arithmetic expression and custom parameter for each individual value of each of those nodes.
Right now I'll revert to creating standalone objects with RailClone Tools and animate them manually. But thanks for the suggestion!
I don't know if this is the right place for a feature request, but it would be nice if things like this could be made easier, maybe with a macro that created (pseudo-)random values based on position (in world-space?) or a checkbox to not re-calculate random values when segment changes. Something as easy as V-Ray's "lock noise pattern" for animations.