The R42 is a New York City Subway car built between 1969 and 1970 by the St. Louis Car Company in St. Louis, Missouri, serving the B (IND/BMT) Division. This fleet of cars were the first to be fully equipped with air conditioning. The R42 fleet is numbered 4550-4949. It was the last 60-foot (18 m) B Division car built for the subway until the R143 in 2001, and the last model class to be built in married pairs. Thereafter, the New York City Transit Authority (NYCTA), for the time being, switched to 75-foot (23 m) cars for the B Division.

On May 9th, 1969, pair 4554-4555 entered service in N service as part of a mixed consist.

Cars in the 4788-4817 and 4820-4839 series are assigned to East New York Yard and run on the J and Z services. Fourteen cars are stored, pending reactivation.

Between 1988 and 1989, R42s underwent overhaul as a result of deferred maintenance in the New York City Subway during the 1970s and the 1980s. 282 cars (4550–4839) were overhauled by Morrisen-Knudsen while the last 110 cars (4840–4949) were rebuilt in-house by the Coney Island Overhaul Shop in Brooklyn. The one minor difference in appearance between the two overhauls was that many cars of the Coney Island version featured the original blue door indicator lights at the ends of the cars, whereas these lights were removed from the Morrison-Knudsen rebuilds.

Cars 4680, 4681, 4714, 4715, 4766, and 4767 were scrapped in 1984, and 4685 and 4726 were scrapped in 2000. On June 6th, 1995 car 4664 and 4665 were involved in a collision on the Williamsburg Bridge. As a result, car 4664 was scrapped in 2000, and car 4665 was mated to R40A 4460, another car left without a mate in the Williamsburg bridge accident. On November 6th, 2007, a consist of R42s was involved in an accident when the motorman attempted to relay an M train south of the Chambers Street. As the R42 fleet is currently being retired, the entire consist was hauled to 207th Street Yard for reefing instead of being repaired, even though only the first two cars suffered major damage and the next four cars were only lightly damaged. 4572-73 (Future NYCTA Museum cars) were used in the famous chase scene in the film The French Connection.

The incoming R160A/R160B order will replace all of the R42 fleet. All of the Coney Island rebuilt R42s as well as many Morrison-Knudsen cars, including the eight involved in the Chambers Street accident mentioned above have been retired and reefed along the Atlantic coast.