Metro-North's Harlem Line, originally chartered as the New York & Harlem Railroad, is an 82-mile (132 km) commuter rail line running north from New York City into eastern Dutchess County. The lower 53 miles (86 km) from Grand Central Terminal to Southeast, in Putnam County, is electrified with a third rail and has two (or more) tracks; north of that point, trains use diesel locomotives on a single track. The diesel trains run as a shuttle on the northern end of the line except during rush hours, when they run as expresses to or from Grand Central. There is also one direct roundtrip to Grand Central on weekends. While the line has traditionally served to bring commuters from Westchester County to jobs in the city, in recent years it has begun to see more "reverse commuting", as Bronx residents use it to reach jobs in Southern Westchester where many stations are within walking distance of city centers. The northern reaches of the line are also close enough to Western Massachusetts to enable residents of part of that region to commute to jobs in the city as well.
With 38 stations, the Harlem Line has the most of any Metro-North main line. Its northern terminal, Wassaic, is the northernmost station in the system. It is the only Metro-North line used exclusively by that carrier (no use by Amtrak, though CSX services freight customers as far north as Mount Vernon) and the only one that uses the entirety of existing track. Metro-North has assigned it the color code blue, used as trim on station signs and spot color on printed timetables. The blue color-coding appears to have started with timetables issued by predecessor New York Central as far back as 1965.