Metro-North’s New Haven Line is the busiest Commuter Railroad Line in the United States. It is operated by the MTA Metro-North Railroad, in a complex arrangement with the State of Connecticut with Metro-North responsible for 35% of costs on the main line. ConnDOT is responsible for the remaining 65% of Main Line costs, and 100% of branch line costs.

The line runs 74 miles from Grand Central to New Haven, Connecticut on the nearly entirely 4 track mainline, that is shared with Amtrak trains on the Northeast Corridor north of New Rochelle. The line is fully electrified with third rail power from Grand Central to a combined section between Pelham and Mount Vernon East, where the line continues under catenary wire to New Haven. All mainline trains use M8 cars, with the older Cosmopolitan M2, M4, and M6 cars retired by 2018.

Train service operates generally every half-hour or better (reduced to hourly due to the COVID-19 pandemic except during rush hour periods) in each direction in a two zoned system with a local train making all stops from Grand Central to Stamford connecting to an express train from Grand Central that has run non-stop from 125th Street, and continues to New Haven running all local stops (some stops north of New Haven have only hourly service), with the half-hourly trains on the outer portion of the New Haven Line running as semi-expresses skipping some stops.