[2] Routes R13 and R14 were discontinued on July 18, 1977 being replaced by the R11, R12, and R15[2] On September 26, 1977, early Saturday morning service on routes R11 & R15's were discontinued, due to new Blue and Orange Line service being introduced during earlier times on Saturday mornings.

Route R11 and R15 were rerouted to operate as a brand new express route between Beltway Plaza Mall (R11)/Greenbelt Center (R15) and New Carrollton station, running primarily along via Greenbelt Road, Cherrywood Lane, Kenilworth Avenue, the Capital Beltway, and John Hanson Highway.

[6] On the other hand, during the time this particular change took place, R12 was truncated to only operate up to the Beltway Plaza Mall, discontinuing service to Goddard Space Flight Center.

Alterations to Greenbelts bus service came back up at the request of the City and Prince George's County.

[10][11] At the time of the proposal, the R12 was suffering bus bunching and scheduling problems due to its long route.

The route was change to operate along Greenbelt Road, Beltway Plaza, Cherrywood Lane, Breezewood Drive, Edmonston Road, Springhill Drive, and back along Cherrywood Lane to Greenbelt station Northbound.

The route would operate vice versa going from Greenbelt station, Springhill Lake Complex, and Beltway Plaza going southbound.

As of a result, route R12's bus stops were moved onto River Road in order to serve the station.

[22] During the Coronavirus disease 2019 pandemic, the R12 was relegated to operate on its Saturday supplemental schedule beginning on March 16, 2020.

[23] However beginning on March 18, 2020, the route was further reduced to operate on its regular Saturday schedule with the supplemental trips not operating and having no Saturday service when WMATA implemented its modified Sunday schedule and regular Sunday service during the weekends.