The 1–50 series was a series of Chicago "L" cars built by the St. Louis Car Company in 1959 and 1960. Unlike cars in the similar 6000 series, which were designed for married pair operation, the 1–50 series cars were double-ended to facilitate single car operation. There was a limited need for single cars, however, so cars 5, 7, 9, 11, 15, 19, 21, 23, 24, and 31 were later rebuilt as married units and were renumbered 61a/b–65a/b.