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

The Pittsburg and Shawmut Railroad is often confused with the similarly named Pittsburg, Shawmut and Northern Railroad from which the P&S had its origins. 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. In fact the two were separate and unrelated companies after their 1916 split.

The main line consisted of approximately 88 miles (140 km) of standard gauge track extending from Brockway, PA  to Freeport, PA. The main shops were located in Brookville, PA.

The Pittsburg and Shawmut Railroad Company began life on July 21, 1903 as the Brookville & Mahoning route of the Pittsburg, Shawmut & Northern Railroad. When the Pittsburg Shawmut & Northern declared bankruptcy in 1915, the Pittsburg & Shawmut Railroad was spun off into a separate entity and was chartered as a separate company in 1916. Like its parent, the P&S was also financially troubled in its early years. The company struggled until corporate fortunes improved with the war mobilization of the 1940s. Coal was the principle commodity for the line for its entire existence. Doodlebugs and passenger trains ran on the route in the early years but had all been eliminated by 1939.

The company acquired a ten mile section of Conrail track running from Sligo, PA to Lawsonham, PA 1989 and reorganized it as the Red Bank Railroad. On December 31, 1991 the company purchased about 110 miles of "low grade secondary" track from Lawsonham to Driftwood, PA from Conrail and organized it as the Mountain Laurel Railroad.