Following the completion of the Dorchester extension, the line became known as the Cambridge–Dorchester line.
It was marked on maps as "Route 1".
After taking over operations in August 1964, the MBTA began rebranding many elements of Boston's public transportation network.
Colors were assigned to the rail lines on August 26, 1965 as part of a wider modernization developed by Cambridge Seven Associates, with the Cambridge–Dorchester line becoming the Red Line.
Peter Chermayeff claims to have assigned red to the line because of Harvard's association with crimson.

In 1968, letters were assigned to the south branches, "A" for Quincy (planned to extend to South Braintree) and "C" for Ashmont. "
B" was probably reserved for a planned branch from Braintree to Brockton.
As new rollsigns were made, this lettering was phased out.
In 1994, new electronic signs included a different labeling, "A" for Ashmont, "B" for Braintree, and "C" for Alewife.
Following the completion of the Dorchester extension, the line became known as the Cambridge–Dorchester line. It was marked on maps as "Route 1". After taking over operations in August 1964, the MBTA began rebranding many elements of Boston's public transportation network. Colors were assigned to the rail lines on August 26, 1965 as part of a wider modernization developed by Cambridge Seven Associates, with the Cambridge–Dorchester line becoming the Red Line. Peter Chermayeff claims to have assigned red to the line because of Harvard's association with crimson.
In 1968, letters were assigned to the south branches, "A" for Quincy (planned to extend to South Braintree) and "C" for Ashmont. "B" was probably reserved for a planned branch from Braintree to Brockton. As new rollsigns were made, this lettering was phased out. In 1994, new electronic signs included a different labeling, "A" for Ashmont, "B" for Braintree, and "C" for Alewife.
Red Line (MBTA)