From: Sydney, Australia
Because someone asked in another post, I'll give my take on it.
Starting in 1941, as the Soviets, you have 4 types of construction support units you can purchase. They are:
* Sapper Battalions
* Sapper Regiments
* Construction Battalions
* RR Construction Brigades
Of these, 2 I never build -- Sapper Battalions and Construction Battalions. Each of these units costs 1 AP to build, and as the Soviets you are constantly short of APs. If you build a Sapper Battalion and a Sapper Regiment they cost the same, but the regiment is more effective, and has a larger construction value for the same AP cost. Hence it makes no sense to build any of the Battalions.
RR Construction Brigades will automatically detach from their Army or Front HQ to repair any broken rail lines in the area. They will do that without any player control, although the player can return them to their controlling HQ during the Soviet turn. This is an extremely effective ability because it reduces your reliance on those scarce NKPS units. If they are attached to an Army HQ that is not locked then they will return to the controlling front HQ once they have finished repairing lines, which isn't so useful -- remember to lock your HQs after you build RR Construction Brigades!!!
Sapper Regiments will not do that, so at first it appears that the RR Construction Brigades are a better buy (the RR Construction Brigades also have a slightly higher construction value, hence build forts a little faster). However, sappers have two extra special abilities, which are:
* They will help destroy enemy forts.
* They can be directly attached to rifle, cavalry and tank corps.
Therefore I always build approximately one sapper regiment for each rifle corps I plan to build, and one for most of the cavalry and mech corps (I don't tend to build them for the tank corps because I don't tend to use tanks to attack forts).
In addition, I build one sapper regiment for each fortified region I build. Note that you have to build them in the controlling army or front HQ first, then transfer them to the fortified region in the next turn, so build them early. Before disbanding the FR I release the sappers back to the HQ. This is all because the construction value of the sappers will be added to the construction value of the FR to assist the FR to build forts.
RR Construction Brigades can't be directly attached to corps, but they will help fortify if they are in the controlling HQ and the HQ is within range. RR Construction Brigades can be attached directly to fortified regions, but I usually use sappers for that instead and let the RR Construction Brigades sit in the HQ and spread out to do their work.