There were once three separate subway operators in New York: the Interborough Rapid Transit Company (IRT), Brooklyn-Manhattan Transit Company (BMT, once known as the BRT, for Brooklyn Rapid Transit Company) and the city-run Independent Subway System (IND). The IRT and BMT were taken over by the city in 1940. In 1967, the systems were reorganized into A division (IRT lines), B division (BMT/IND lines) and C division (work trains).

The IRT has the narrow cars, because the original 1904 subway line was constructed to the dimensions of those that ran on the Manhattan elevated lines (in 1903, the IRT acquired the Manhattan Railway, which ran all four of the Manhattan els). Both the engines and the cars were smaller and weighed as little as possible; when the els were being built, folks were afraid that larger locomotives would fall from the elevated structure, and so the companies adopted smaller ones.

The Brooklyn els were built to the specifications of the surface railroads (e.g. the LIRR). When the Brooklyn subways were being built the same dimensions were used The city decided on the larger cars for the IND, as they carried the most passengers; it would also allow for joint BMT/IND service down the line. Interestingly, all IRT stations constructed after the 1913 Dual Contracts were built to handle the larger trains, but they don't run due to the sharp curves on the older tunnels!