From: Birkerod, Denmark
Had the same problem, which is why I've switched to sapper regiments instead of RR brigades to lend a helping shovel with the digging of trenches.
I think it's WAD (though as usual, I could just as easily be wrong), since non-combat SUs are outside the control of the player.
It might (*might*, mind you) have something to do with most armies already being at their CP limit, thus the AI sends them to the Fronts instead.
Haven't really looked deeper into the problem since I switched to sappers instead though, so there might be some intricate way around the issue.
But I've found (in my admittedly limited experience) that a single RR brigade in each Army is more than enough to repair the railroads, at least during 41-42.
It only becomes an issue if you're also relying on the brigades to lend construction-support to your CUs.
Hence the sappers. Least they stay where intended instead of wandering off into the countryside, admiring their handiwork.
"Something is always wrong, Baldrick. The fact that I'm not a millionaire aristocrat with the sexual capacity of a rutting rhino is a constant niggle"
- Edmund Blackadder