R21 (New York City Subway car)
 The R21 was a New York City Subway car built by St. Louis Car Company in 1956-1957. The R21 was similar to the R17 except that it featured windows of a slightly different design. Two different propulsion system were used Westinghouse (WH) and General Electric (GE).The first set of R21s was placed in service on the 1 train on November 7 1956.