The R12s were retired in the early 1980s due to service reductions prior to the delivery of the R62 fleet in the mid-1980s, and the final train of R12s ran in September 1981.

The two car types were very similar to each other, except that the R12s were smaller since they were built to meet A Division specifications.

The R12s were the first "R" type contract order built for the A Division (referring to the practice of naming a car class by the letter "R" – which stands for rapid transit – followed by a number derived from the actual contract number).

After the merger of the IRT, BMT, and IND in 1940, all subsequent subway car orders would follow the R contract.

The cars were delivered in two-tone gray scheme with orange stripes, then a solid bright red, and finally, repainted into the MTA's silver with blue stripe scheme.

One particular assignment included all GE-powered cars being heavily modified (by deactivating the dynamic brakes and cutting out the third (parallel) notch on their controllers) and sent to the 8 (Third Avenue elevated line) in the Bronx during August 1969 and running there until that route's closing on April 29, 1973, whereupon they were permanently removed from passenger service and relegated to work service after only a little under 25 years in passenger service.

As newer cars started to run on the MTA's many lines, the R12s were in a state of disrepair.