List of New York City Subway inter-division connections
The New York City Subway was formed from three different systems, the IRT, BMT and IND. For operational purposes, the IRT is A Division and the BMT and IND make B Division; however, common usage calls the three systems divisions. Over the years, various connections have been built to allow trains of one division to operate on the lines of another.
For transfers, see List of New York City Subway transfer stations.
Revenue connections 
Because the IRT system uses narrower tunnels and cars than the BMT and IND, the only possible inter-division revenue connections are between BMT and IND lines. The following connections have been built:
- October 30, 1954: IND Culver Line (F train) (originally called the Smith Street Subway) extended over BMT Culver Line; original routing disconnected and became the Culver Shuttle until 1975
- December 1, 1955: 60th Street Tunnel Connection (R train) between the IND Queens Boulevard Line and the BMT 60th Street Tunnel
- April 29, 1956: IND Fulton Street Line (A train) joined to BMT Liberty Avenue Elevated (part of the Fulton Street Elevated, but disconnected from the BMT system and so is only used for IND service)
- November 26, 1967: Chrystie Street Connection between the IND Sixth Avenue Line and the BMT Nassau Street Line (via the Williamsburg Bridge – M train) and BMT Fourth Avenue Line (via the Manhattan Bridge – B D trains)
- October 29, 1989: connecting tracks between IND 63rd Street Line and BMT 63rd Street Line (not generally used for revenue service unless a re-route of the R train is required north of 57th Street – Seventh Avenue on the BMT Broadway Line)
Non-revenue connections 
These connections mainly take place in yards:
- Crossover tracks (with third-rails) between northbound IRT Flushing Line and BMT Astoria Line at Queensboro Plaza (the Flushing Line's only connection to the rest of the system, originates from when both lines north of Queensboro Plaza were dual-operated)
- IRT New Lots Line and BMT Canarsie Line both have a single non-electrified track into the Linden Shops. The track leaves the New Lots Line at Junius Street and joins the track from Canarsie line south of Livonia Avenue before arriving in the Linden complex
- IRT Broadway – Seventh Avenue Line and IND Eighth Avenue Line both have tracks to 207th Street Yard
- IRT Jerome Avenue Line and IND Concourse Line both have tracks to Concourse Yard
- Since October 30, 1954, the IND Culver Line has gone into Coney Island – Stillwell Avenue, where it connects to the BMT Brighton Line, BMT Sea Beach Line and BMT West End Line. For a short time in the late 1960s the NX service passed through Coney Island – Stillwell Avenue to and from the Sea Beach and Brighton Lines, but no service has passed through from the Culver Line.