Harold Interlocking is a large railroad junction located in New York City. It is the busiest rail junction in the United States. It serves trains on Amtrak's Northeast Corridor and the Long Island Rail Road's Main Line and Port Washington Branch, which diverge at the junction. Reconstruction work on Harold Interlocking started in 2009, as part of the East Side Access project to bring LIRR service to Grand Central Terminal. As part of the project, two tunnels for Northeast Corridor trains bypassing Harold Interlocking are being built to address congestion problems and occasional accidents.