More details about railway engineers organization:
In 1941 Soviet railway troops consisted of three principal parts:
1. Special Corps of Railway Troops. The Corps was an RKKA formation but was under operational control of the NKPS (civil ministry of transport). The Corps consisted of 5 railway brigades with 27 railway construction battalions, 3 railway bridge battalions, 2 mechanization battalions (units with heavy machinery), 5 mechanization companies, 2 railway operating companies, 5 training battalions. Total in 5 brigades – 53 572 men. Also 7 railway operating regiments, total 14 430 men. All combined 68 002 men. By the start of 1941 the Corps was deployed mostly in the Far East where it built the Baikal-Amur railroad. In the spring of 1941 the Corps HQ and 3 brigades (1, 4, 5 Brigades) were transferred to Ukraine, 3 and 7 Brigades stayed in the Far East. After mobilization organization and strength remained basically unchanged to the end of 1941. In September 1941 the Corps was disbanded as an operational command, thereafter brigades operated independently.
2. Separate railway formations under control of RKKA Transportation Directorate. Before mobilization they consisted of 8 brigades, 8 regiments and other units with a total strength of 29 260. With the start of mobilization each regiment was converted to a replacement regiment and also formed a new brigade HQ. So after mobilization there were 16 (8+8) railway brigades and smaller units: 42 reconstruction battalions, 15 bridge battalions, 16 mechanization battalion, 25 railway operating companies, 1 suspension bridge battalion, 8 front railway depots, 2 central railway depots, 7 replacement railway regiments. Each brigade consisted of 2-3 railway reconstruction battalions, 1 bridge battalion, 1 mechanization battalion, 1-2 operating companies. Total strength after mobilization – 97 277 men. By the end of 1941 1 Brigade HQ, 7 battalions, 1 training regiment, 1 operating company and 1 front depots were additional formed which brought the number of brigades to 17. It must be mentioned that separate railway brigades and brigades of the Railway Corps had different organization and different TO&Es and the second were about twice as large.
3. Special formations of the NKPS – various repair trains, workshops and depots with civilian workers. The performed tasks similar to 1) and 2) but with more emphasize on special operations (recovery of signal lines and water supply, repair of rolling stock etc).
On 3 January 1942 railway troops were reorganized by a special GKO order. All RKKA railway troops (17 brigade HQs, 8 replacement regiments, 44 railway reconstruction battalions, 17 bridge battalions, 17 mechanization battalions, 26 railway operating companies, 9 front railway depots, 3 central depots and Transportation Military School) with 104384/76816 authorized/actual strength were transferred to the NKPS. In addition the Red Army was to provide replacements for bringing those units to full strength and to form 5 additional brigades with 20 reconstruction battalions, 5 bridge battalions, 6 mechanization battalions, 11 operating companies, and also 2 replacement regiments. In addition to 5 brigades of the former Special Railway Corps already under NKPS control that was to make 17+5+5=27 railway brigades with a total strength of 222 thousand men. It must be mentioned that 5 brigades of the former Railway Corps retained their special organization until new unified TO&Es were issued in 1943. A Main Directorate of Military Reconstruction Work (GUVVR) was created inside the NKPS to control railway construction/reconstruction operations. It included the Railway Troops Directorate formed from the former HQ of Railway Corps which was to command all railway engineer units. GUVVR also adsorbed NKPS civil formations (total 34 800 men) which were reorganized as military units and their workers received status of military personnel. They however retained their special organization and were not included in railway brigades. Each Soviet Front received a special HQ – Military Reconstruction Work Directorate (UVVR) which was to control railway construction/reconstruction and railway troops in the front area. UVVR typically commanded one or several railway brigade, each usually operating on a separate railway line.
This principal organization remained basically unchanged to the end of the war, although railway engineer force was expanded and additional new units formed. By the end of 1942 there were 30 railway brigades, 2 railway operating brigades and misc. units with 225600/196300 authorized/actual strength (of them 112 500 in the operational forces on the front). By the end of 1943 – 250301/253492 authorized/actual strength in railway troops. Also 60500 men in the NKPS formations (186 various units, mostly repair trains) In 1945 – 35 railway reconstruction and 2 operating brigades, 19 operating regiments, 9 replacement regiments, 110 reconstruction and 40 bridge battalions, 30 mechanization battalions, 30 signal recovery battalion, 3 truck battalions etc. Total 271 652 men in May 1945.
In February 1943 new standard TO&Es for the railway brigade were issued. The brigade was to consist of HQ, 4 railway reconstruction battalions, one bridge battalion, one mechanization battalion and one railway operating company. Later on a signal recovery battalion, carpenter battalion and an additional operating company were added to the brigade structure, a water supply company included in each mechanization battalion. That brought the brigade authorized strength to 6306 men (see the organizational chart here