From: Denver Colorado
Okay, there are two types of rail repair units for each side.
The basic type you worry about is the type you control. So let's wait on those and deal with the type you do not control. Various RR construction (for Soviet) and many designations of "construction" unit (for the Germans/Romanians/Finns) pop out of HQs onto the map and automatically repair rail hexes. These un-controllable units won't repair what you want them to unless it's by rare accident, and they will repair whatever they work on very slowly (so you often see 2 on one rail hex), at least compared to the repair units you control.
The rail units you control you must manage yourself. They can generally repair 2 to 5 hexes per clear-weather turn. They are restricted practically by movement points most often, but in the Baltic Rail zone, Axis rail repair is 1 per hex to convert rather than the usual 3 per hex. So there you sometimes run into the problem of being limited to repairing rail hexes within 6 or 8 (or whatever the limit, I forget) of where your existing rail head was at the start of the current turn (i.e., what you fixed last turn).
Summer 2017-Playing: D-Day at Omaha Beach, Advanced Squad Leader,
Reading: Kampfgruppe Walther & Panzerbrigade 107 (Magnificent). Lots of Osprey stuff.
Rulebooks: ASL (always ASL)