The Galena & Southern Wisconsin Railroad was incorporated in 1857 as combination of two separate Galena & Wisconsin Railroads, one in Illinois and one in Wisconsin. Its main line between Galena, IL, and Platteville, WI, at 21.2 miles in length, was constructed in 1874 with a 3-foot narrow gauge, a more feasible option when compared to the costs of standard gauge. Notable along this route is one of Wisconsin's few railroad tunnels, Buncombe Tunnel, which was built in 1875 on the Illinois/Wisconsin border near Buncombe proper, in Lafayette County. A branch line at Ipswitch was built north to Rewey, a distance of 8.5 miles, in 1877.
In 1880, the G&SW was merged with the nearby Chicago and Tomah Railroad to form the Milwaukee and Madison Railroad, a subsidiary of the Chicago and North Western Railroad. Later that year, the branch line to Rewey was extended north to Montfort, where it connected with a C&NW line at Montfort Junction. The railroad was further merged into the Chicago, Milwaukee and Northwestern Railway in March of 1881.
At the time, the C&NW was one of the larger railroad operations in the state of Wisconsin, and had amassed a wide collection of narrow-gauge railroads within the state. In 1882, it re-gaugued most of these narrow gauge lines to standard gauge, the former G&SW included. This also included widening of Buncombe Tunnel to accommodate the additional track/loading width.
A spur from Millbrig, WI, (just south of Buncombe Tunnel) to Hazel Green was built in 1907, a distance of 2.7 miles. By this time the CM&N had been fully absorbed into the C&NW; the former G&SW was then known as its Platteville Subdivision.
The first abandonment of the former G&SW came in 1938, when its line between Galena and Benton, WI, was abandoned. This also meant abandoning Buncombe Tunnel (which still stands today). The remainder of the line saw diminishing traffic, with the last passenger train in 1955. Mining in the area of Platteville came to an end in 1979, and with it, the final vestige of railroad traffic; the rest of the line was ultimately abandoned a year later in 1980.