The Pittsburgh Southern Railway was a railroad system that was located in the Commonwealth of Pennsylvania, United States.

This American railroad system was built in March 1879 through the merger of the 3 ft (914 mm) narrow gauge Pittsburgh Southern Railroad (which was the 3 ft 4 in (1,016 mm) narrow gauge Pittsburgh, Castle Shannon and Washington Railroad[1] from July 1877 to April 1878), Pittsburgh Railroad, and Washington Railroad.

An attempt to use the 4 ft 8+1⁄2 in (1,435 mm) standard gauge Little Saw Mill Run Railroad as a substitute connection to Pittsburgh using dual gauge track led to the Castle Shannon Railroad War of 1878.

[2][3] In 1883, it was converted to 4 ft 8+1⁄2 in (1,435 mm) standard gauge.

After a year, it was purchased by the Baltimore and Ohio Railroad on November 20, 1884, and they reorganized them as the Baltimore & Ohio Short Line Railroad.