The Waterbury Branch is a branch of the Metro-North Railroad's New Haven Line, running north from a junction in the Devon section of Milford to Waterbury, Connecticut. Originally built as the Naugatuck Railroad, it once continued north to Winsted. The part north of Waterbury is now leased from CTDOT by the Railroad Museum of New England, which operates excursion trains from Thomaston station through their operating subsidiary Naugatuck Railroad ; this name was chosen in homage of the original railroad. The trackage ends in Torrington, but Metro-North service on the branch ends at Waterbury. There are conceptual plans to extend service from its current terminus in Waterbury to Hartford via Bristol and New Britain. Currently, riders that want to continue to New Britain and Hartford have to transfer to an express bus operated by CTtransit at Waterbury. All trains on this branch operate as shuttles between Waterbury and Bridgeport.

Metro-North Railroad, trading as MTA Metro-North Railroad, is a suburban commuter rail service operated by the Metropolitan Transportation Authority (MTA), a public authority of the U.S. state of New York. Metro-North serves the New York Metropolitan Area, running service between New York City and its northern suburbs in New York and Connecticut, including Port Jervis, Spring Valley, Poughkeepsie, Yonkers, New Rochelle, Mount Vernon, White Plains, Southeast and Wassaic in New York and Stamford, New Canaan, Danbury, Bridgeport, Waterbury, and New Haven in Connecticut. Service in Connecticut is operated under contract with the Connecticut Department of Transportation. Metro-North also provides local rail service within the New York City boroughs of Manhattan and the Bronx.