The IND was the New York City Subway division created by the city as an independent public system.
IND stands for Independent Subway System, and the city opened its first IND routes in the 1930s. Unlike the privately built IRT and BMT systems, the IND was planned and operated by New York City before the three systems were unified under public control in 1940.
The IND helped expand service into areas that had been underserved and used lettered routes such as the A, C, E, and F. Its stations and tunnels were generally built to accommodate larger cars than the older IRT infrastructure, while BMT equipment and clearances followed different standards.
The abbreviation SIRT refers to the Staten Island Rapid Transit system, not the IND. The three historical divisions still matter because differences in car dimensions, track layout, and route lettering or numbering continue to shape subway operations.