This is from the scenario description document.
These units are for railroad repair and were designed specifically for easier playability. They can move only by rail, and will repair damaged rail lines nearest their location. Repairs take place automatically between turns, and require no action by the player. A bautrupp unit need not be directly adjacent to broken rail lines in order to make repairs, just nearby or in the vicinity. It should be sufficient to keep these units several hexes behind the front and out of harms way. The game engine repairs rail hexes each turn automatically, and it will tend to focus on broken rail lines near any bautrupp units [embarked or not]. Therefore, the player can easily channel rail repairs to specific important areas. Because it is a large map you may not notice all repaired hexes as they can occur anywhere.
These units will all reconstruct, but there is no need to put them in harm's way so you really shouldn't lose any of them. Note that if you hit the '.' [period, dot] key on the keyboard, you can easily cycle thru these units [if they are embarked as they should be].
The maximum number of Axis Rail Repairs is set to twelve, although during the Axis Supply Collapse the maximum number is reduced to three [from November 16th, 1941 to March 1st, 1942]. During this time period it may be best to concentrate all Bautrupp units in one area in order to see any effect.