Focusing rails do not have the necessary precision usually. Microscope stage would do it and some use that. More common is a linear stage with micrometer such as Newport 423/433. Used one from eBay will not be overly expensive. Some manufacturers of arca rails like Hejnar have adapters for Newport and also now offer a focusing rail with micrometer, inspired by the linear stages I presume.
I agree with Torbjörn that focus rails usually are not all that precise, in particular for really close-up work. One of the problems is that most camera systems are not balanced around the center of gravity, and therefore the camera/lens are not perfectly aligned until the stage is locked at each new position. The RRS rail is no exception, but it is nicely made and pretty compact. That locking requirement alone adds two steps (unlocking to move, locking after moving) to the stacking workflow. That gets tedious with large numbers of frames.
A Stackshot rail is much more solid in that respect, and the automation makes it a joy to work with it, but it's not a cheap solution (and it's overkill for most occasional users). A benefit of good Stacking software is that it also takes care of some of the inaccuracies (repositioning, magnification, exposure).
Cheers,
Bart
