From: Chicago, Illinois
ORIGINAL: jon matthews
A potential disadvantage is they can either assist in digging in or repair rail, but not both. And if you want them to repair rail, note this from 188.8.131.52, page 196 in the EBook "For human players only, there is a limit to the distance that the automated rail repair units will operate from the HQ unit that they are attached, which is based on command range (7.6.4). For example, if a construction battalion is assigned to a Corps HQ unit, it can only repair rail line hexes up to 5 hexes from that HQ, but the same construction battalion attached to a High Command HQ unit (e.g. OKH or STAVKA) could operate up to 90 hexes away."
Having them in shorter-range Corps, though, can be an advantage. The reason is that OKH tends to backfill every spur and siding close to Poland before working on lines nearer to the front. The Corps Repair units, because they can only go 5 hexes, will work on rails near the front.