For supply, more and better run RRs. They had tons of ammo, and munitions sitting in warehouses, while the guns and men on the front didn't get their ordered/needed items. If they had more RR lines/cars and engines, and people who weren't mooks, running them. They could have had their troops at the front fighting at something close to 100% efficiency. This also would have allowed them to actually use their numbers pre 1916.