The Old Colony Lines are a pair of branches of the MBTA Commuter Rail system, connecting downtown Boston, Massachusetts with the South Shore and cranberry-farming country to the south and southeast. The two branches operate concurrently for 10 miles (16 km) via the Old Colony Mainline from South Station to Braintree station. The Middleborough/Lakeville Line then winds south through Holbrook, Brockton, Bridgewater, Middleborough, and Lakeville via the Middleborough Main Line and Cape Main Line. The Kingston Line heads southeast to serve Weymouth, Abington, Whitman, Hanson, Halifax, and Kingston by way of the Plymouth branch. Limited service to Plymouth was provided prior to April 2021 but was cut due to low ridership and budget constraints. The Greenbush Line, which was also part of the Old Colony Division, was reactivated in 2007 as a separate project.

The MBTA Commuter Rail system serves as the commuter rail arm of the Massachusetts Bay Transportation Authority's (MBTA's) transportation coverage of Greater Boston in the United States. Trains run over 394 mi (634 km) of track to 134 stations. It is operated under contract by Keolis, which took over operations on July 1, 2014, from the Massachusetts Bay Commuter Railroad Company (MBCR).