A new kind of logistics
By 1861 the United States had a large but fragmented railroad network. Armies quickly learned that control of junctions, bridges, depots, and rail corridors could determine whether troops were supplied or stranded.
Railroads did not replace marching, wagons, or waterways, but they increased the distance over which large armies could be supported and reinforced.
North and South
The Union possessed a larger and denser rail network, greater locomotive and car-building capacity, and more industrial ability to replace rails and equipment. Southern railroads faced shortages, incompatible gauges in some areas, destruction, and increasingly difficult maintenance.
Railroads as military targets
Bridges, tracks, stations, and junctions became military objectives. Armies destroyed railroad infrastructure to interrupt an opponent's logistics, while military railroad organizations repaired and operated captured lines.
Why it mattered afterward
The war demonstrated the strategic value of coordinated transportation. In the decades that followed, railroad construction accelerated and helped bind regional markets into a much larger national economy.