For large construction projects, civilian workers were also contracted, for example, up to 1,400 carpenters were employed to build the Etowah and Chattahoochee Bridge. The operating units managed the provision and proper use of operational materiel and services. The construction units had the task of building new lines, repairing destroyed railway facilities or even destroying them themselves. They were divided into railway operating units as well as construction units with sub-units for line and bridge building.
During the war this branch of the army grew to about 25,000 men. To begin with, McClellan formed a construction corps from ordinary soldiers, but he soon recognised that the lack of training of these troops for technical work meant that a specially organised corps was needed within the Union Army for technically trained civil engineers and workers. In the American Civil War, unlimited authority over all railway lines in the North was given to General McClellan.
In many countries, however, there were little or no military units of this type.
Originally these were known (at least in the German-speaking areas of Europe) as field railways. The establishment of railway troops by the great powers followed the emergence, rapid growth and rising importance of the railway network, when the advantages of the railway for the transport of troops, heavy weapons and supplies became recognised.