The Pittsburg and Shawmut Railroad (reporting mark PSR), also known as the Shawmut Line, was a short line railroad company operating passenger and freight service on standard gauge track in central and southwestern Pennsylvania.

Further adding to the confusion is the fact that both were nicknamed the Shawmut Line, both operated in roughly the same geographic area, and both used similar diamond logos during their history.

When the PS&N declared bankruptcy in 1905, the B&M was spun off into a separate entity and was renamed in 1909 due to confusion with the Boston and Maine Railroad's initials.

The company acquired a ten-mile (16 km) section of Conrail track running from Sligo to Lawsonham in 1989 and reorganized it as the Red Bank Railroad.

On December 31, 1991, the company purchased about 110 miles (180 km) of the Low Grade Secondary track from Lawsonham to Driftwood, Pennsylvania from Conrail and organized it as the Mountain Laurel Railroad.