The West Side Line, also called the West Side Freight Line, is a railroad line on the west side of the New York City borough of Manhattan. North of Penn Station, from 34th Street, the line is currently used by Amtrak passenger service heading north via Albany to points as far north as Montreal, and west to Chicago. South of Penn Station, a 1.45-mile (2.33 km) elevated section of the line, abandoned since 1980, has been transformed into an elevated park called the High Line, which opened in sections between 2009 and 2014.

The line as originally constructed in 1849 was primarily at-grade, much of it running along streets. Its southern portion was replaced in the 1930s, with an elevated portion up to 35th Street, and a below-grade portion on a new alignment up to 59th Street. At about the same time, the portion from 72nd Street to 120th Street was covered to form what is now called the Freedom Tunnel. In the 1980s, the elevated portion south of 35th Street was abandoned, and the new Empire Connection tunnel connected the remaining portion to Penn Station. The 60th Street Yard, which occupied the space between the below-grade sections, was redeveloped into Riverside South and the tracks covered in the late 1990s and 2000s.

The West Side Line was built by the Hudson River Railroad, which completed the forty miles (64 km) to Peekskill on September 29, 1849, opened to Poughkeepsie by the end of that year, and extended to Albany (Rensselaer) in 1851. The city terminus was at the junction of Chambers and Hudson Streets; the track was laid along Hudson, Canal, and West Streets, to Tenth Avenue, which it followed to the upper city station at 34th Street. Over this part of the right-of-way, the rails were laid at grade along the streets, and since the Hudson River Railroad's regulations did not allow locomotives to draw cars through streets, the cars were drawn by a dummy engine. While passing through the city the train of cars was preceded by a man on horseback known as a "West Side cowboy" or "Tenth Avenue cowboy" who gave notice of its approach by blowing a horn.

At 34th Street, the right-of-way curved into Eleventh Avenue, the dummy engine was detached, and the regular locomotive took the train. As far as 60th Street, the track was at street level. The first cut was at Fort Washington Point. The railroad crossed Spuyten Duyvil Creek on a drawbridge; a fatal wreck occurred there on January 13, 1882, when the Atlantic Express, stopped on the line, was rear-ended by a local train, telescoping the last two palace cars, where the stoves and lamps were upset and ignited the woodwork and upholstery.

In 1867, the New York Central Railroad and Hudson River Railroad were united by Cornelius Vanderbilt, being merged in 1869 to form the New York Central and Hudson River Railroad. The railroad acquired the former Episcopal church's St. John's Park property and built a large freight depot at Beach and Varick streets, which opened in 1868. The tracks south to Chambers Street were then removed. In 1871, the Spuyten Duyvil and Port Morris Railroad (Hudson Line) opened, and most passenger trains were rerouted into the new Grand Central Depot via that line along the northeast bank of the Harlem River and the New York and Harlem Railroad (Harlem Line), also part of the New York Central system. The old line south of Spuyten Duyvil remained for freight to the docks along Manhattan's west side and minimal passenger service to the West Side station on Chambers Street (used until 1916).

As the city grew, congestion worsened on the west side. Eventually, plans were drawn up for a grade-separated line. The West Side Elevated Highway was built with the line's grade separation in the 1930s. Work on the highway – named for Manhattan Borough President Julius Miller, who championed it – began in 1925, and the first section was dedicated on June 28, 1934. This included a new, elevated eight-track freight terminal called St. John's Terminal, located several blocks north of the old one at St. John's Park, with its southern edge at Spring Street. North of there, an elevated structure (the present-day High Line) carried two tracks north on the west side of Washington Street, curving onto the east side of Tenth Avenue at 14th Street, then crossing Tenth Avenue at 17th Street and heading north along its west side. Just south of the Pennsylvania Station rail yard at 31st–33rd Streets, the line turned west on the north side of 30th Street, then north just east of the West Side Highway. The northernmost bridge crossed 34th Street, and a ramp took it back to Eleventh Avenue south of 35th Street. The elevated line was built through the second or third floors of several buildings along the route; others were served directly by elevated sidings.

In 1937, the tracks along Eleventh Avenue were bypassed by a below-grade line, passing under the 35th Street intersection and running north just west of Tenth Avenue before slowly curving northwest, passing under Eleventh Avenue at 59th Street and rejoining the original alignment. There were three sections that remained open, one at 37th Street, one at 45th Street, and one at 49th Street. The one at 37th Street was covered over in the mid-2010s, but the openings at 45th and 49th Streets remain to this day.

Around the same time, New York City Parks Commissioner Robert Moses covered the line from 72nd Street north to 120th Street with an expansion of Riverside Park. His project, called the West Side Improvement, was twice as expensive as the Hoover Dam and created the Henry Hudson Parkway, as well as a railroad tunnel under the park. The large 60th Street Yard served as the dividing point between the two-track realignment and a wider four-track line to the north. North of 123rd Street, the line became elevated between the Henry Hudson Parkway and Riverside Drive before returning to the surface and crossing under the Parkway to its west side near 159th Street. It continues along the shore of the Hudson River to the Spuyten Duyvil Bridge, a swing bridge across the Harlem Ship Canal (Spuyten Duyvil Creek), before merging with the Hudson Line just north of the bridge.