21 is a streetcar (or trolley) in Fort Collins, Colorado, listed on the U.S. National Register of Historic Places.

By the end of service in 1951, the Fort Collins Municipal Railway was the last streetcar system in the United States to be using any Birney cars.

In 1977, the Fort Collins Junior Women's Club proposed giving 1919-built streetcar 21 a cosmetic restoration, as the car had become dilapidated from years of exposure.

[6] With this goal, the non-profit Fort Collins Municipal Railway Society (FCMRS) was formed, on March 31, 1980.

The heritage streetcar operation would use the same name as the former streetcar-transit system, the Fort Collins Municipal Railway.

The 600 Volts DC that powers the car's electric motor is collected from the overhead trolley wire via a trolley pole on the roof; only one of the two poles is in use at a time, depending on the direction of travel.