The Northeast Corridor (NEC) is the busiest passenger rail line in the United States by ridership and service frequency.  The route is fully electrified and serves a densely urbanized string of cities from Washington, D.C., in the south through Baltimore, Wilmington,Philadelphia, Trenton, Newark, New York, New Haven, and Providence to Boston. It also has branches connecting Philadelphia with Harrisburg, Pennsylvania; New Haven with Hartford, Connecticut, and Springfield, Massachusetts; New York City with Albany, New York, and several other commuter destinations. The busiest passenger rail station in the United States is Pennsylvania Station in New York, the central hub of the Northeast Corridor.

The NEC is immediately identified by the use of overhead wires and high speed rolling stock. Mostly operated and owned by Amtrak, the NEC offers the only true high-speed rail service in the United States, Amtrak's Acela Express, as well as lower-speed conventional pasenger trains. Freight trains also use the tracks. Several commuter rail agencies provide local service along the Northeast Corridor, some electrified and some diesel-powered. These rail networks are MARC in Maryland and Washington, D.C., SEPTA in Pennsylania and Delaware, NJ Transit in New Jersey, Metro-North in New York and Connecticut, Shore Line East in Connecticut, and MBTA in Massachusetts and Rhode Island.

Interstate 95 closely parallels the Northeast Corridor mainline for its entire length. The mainline can be seen from portions of Interstate 95. Indeed, I-95 so closely parallels the rail line that it even sometimes takes the same curves as the rail line, especially in Connecticut.