On June 1, 1940, the City of New York acquired the two subway systems it didn't already own - the IRT (Interborough Rapid Transit Co.) and the BMT (Brooklyn-Manhattan Transit Corp.) - and consolidated them with the city-owned IND (Independent Subway System). It was readily apparent that the city's fleet of aging subway cars was desperately in need of replacement, and immediately after World War II, management began to develop a new car that would be standard throughout the system and incorporate the latest advances in subway design. This effort was complicated by the fact that portions of the IRT had tighter clearances than the IND and BMT, so all future designs would incorporate a shorter, narrower IRT version.
Beginning with contract R-10, and IRT-sized contract R-12 delivered in 1948, the new cars featured welded steel bodies, flourescent lighting that made them considerably brighter than prewar cars, and seating made of velon, a new plastic material that replaced the rattan seating of older cars. Double doors made for faster loading and unloading; for standees, poles replaced the "strap hanger" grab handles of prewar cars. The arrival of the first R-12 cars, ferried across the East River on a barge from the Hoboken rail terminal, was heralded by a New York City fireboat shooting plumes of water. Banners on the barge proclaimed "New Yorkers, Look! Your New Subway Cars!"
A major mechanical improvement on the postwar cars was a new type of brake system known as Straight Air Motor Car Electric-Pneumatic Emergency (SMEE). The new system combined ordinary air brakes with dynamic braking, in which a car's electric motors, by having their polarities reversed, were converted to generators in order to slow the car. This significantly reduced brake shoe wear and maintenance costs.
Starting with the R-12, the postwar IRT cars were known as the SMEE fleet. The 100 R-12s from American Car & Foundry were soon supplemented by near-identical R-14s and several similar types. By 1964, when the last SMEE cars arrived on the system, the SMEE fleet numbered 2,860 fully-compatible cars. Up through the late 1980s, it was common to see SMEE cars of various vintages and paint schemes combined in a single train.
For 2019, the R-12 returns to the RailKing lineup in each of the schemes it wore over four decades of service: the as-delivered two-tone grey with orange stripes, the later bright red, the final in-service MTA silver with blue stripe, and yellow work train use after retirement.