When it comes to rail repair, no one strategy is right. However, I think I have seen some players not maximizing their rails in the center. The trick is not to double up (maybe only if you face the absolute worst sort of Comrade Robinski, and even then I am not sure that 1 more hex a turn is worth it...), but to use what you have wisely.
Here is an image of what I do - it is not perfect, there really is no right or wrong. However, it is designed to support the sort of advance that I want to have in my "mobility" thread.
The Center has two repair units. The top helps out the AGN repair unit until they split at the initial swamp - then the top on hugs the rail line along the Baltic Zone (excepting the one place it must dip down) to speed up repair. By turn 14, with proper breakthroughs, it can be in Vyazma and the upper part of the Center can have fairly decent supply.
The lower of the Center units does not drive to Minsk, but rather the long straight rail line just north of the Mashes. By turn 14 it can be beyond Gomel and heading east.
This has allowed me to get more supplies for a Moscow Drive (or threat there), not have to use HQ buildup for the tanks in near Vyazma, and have nice parallel lines to link up north south during mud/snow preparing for emergencies.
Really, this is not perfect, it is just the way I do it...




