Railroad MOW sheds make easy scratchbuilding projects for a model railroad.
In the article “A tale of two sheds” author Harold W. Russell describes two prototype railroad maintenance-of-way (MOW) sheds and includes scale drawings. Use the drawings and the article as a basis for a scratchbuilding project on your model railroad.
Yes, it is very common for railroad buildings to be prefabricated by the railroad or by contractors and shipped to location.
Railroads have done this for over a hundred years. Many wooden buildings were moved on flatcars, including very large stations. Of course, tunnels and bridges would limit the size or range of the move. Sometimes a building would be sliced in half first.
Regarding the small wooden building, are those lifting eyes at the four corners of the roof? Did the prototype prefab these and load them onto a flatcar to drop off along the track as needed?