125th Street (IRT Lexington Avenue Line)

125th Street is an express station that has four tracks and two island platforms. It is the northernmost Manhattan station on the IRT Lexington Avenue Line of the New York City Subway. Located at Lexington Avenue and East 125th Street (also known as Dr. Martin Luther King, Jr. Boulevard) in Harlem, it is served by the ' and ' trains (all times), and the  train (all times except late nights). The station lies one block east of the Harlem–125th Street station on the Metro-North Railroad. A proposed northern extension of the Second Avenue Subway will connect with this station.

IRT Lexington Avenue Line platforms
The station is unique in design. It is a bi-level station, with an island platform on each level, but is not configured in the standard express-local lower-upper configuration (which is used in most stations south of this point). Instead, the upper platform serves northbound (uptown) trains and the lower level serves southbound (downtown) trains. North of the station, just after crossing the Harlem River, the line splits into the IRT Jerome Avenue Line (heading north) and the IRT Pelham Line (heading east). On the lower platform, each track comes from one line, and a flying junction south of the station allows trains to choose the local or express track. On the upper platform, the west track comes from the local track, and the east track comes from the express track; a flying junction north of the station splits the lines.

An unused track ends on a bumper block just south of upper level platform. The unused track merges with uptown local track at its southern end. Another unused track connects from the downtown express track to the downtown local track, just south of this station. A third unused track continues from the uptown express track and merges to the uptown local track, just north of this station.

There is an active tower at the north end of the upper platform; it is a satellite to the tower at 42nd Street – Grand Central, which controls the entire length of the Lexington Avenue Line. This station's renovation was completed in 2005.

Second Avenue Subway
The planned northern terminal for the Second Avenue Subway would be built below and at right angles to this station, along 125th Street. Phase 1 of the Second Avenue Subway from 63rd Street to 96th Street is under construction as of October 2009. This station is part of Phase 2 from 96th Street to 125th Street. The planned configuration is an island platform with two tracks as well as a third track served by a side platform like the 34th Street terminal station under construction for the 7 Line Extension.