Construction began on May 20, and the first section to Waltham opened on December 20, 1843, operated by the Charlestown Branch until May 1, 1844.

Further sections opened to Concord June 17, 1844, Acton October 1, 1844, Shirley December 30, 1844, and Fitchburg March 5, 1845.

[citation needed] The original Charlestown terminal was southwest of City Square, west of the Warren Bridge (42°22′12″N 71°03′47″W / 42.370°N 71.063°W / 42.370; -71.063).

[7] Planning to eliminate the eleven remaining grade crossings in Somerville, five of which were on the Fitchburg Route mainline, began in 1900.

[8] In 1906, the city engineer proposed to raise 1.8 miles (2.9 km) of the line between Beacon Street and Somerville Avenue to eliminate the five level crossings, but that scheme was not adopted.

[9] The Somerville Avenue crossing of the Fitchburg Route and the parallel Grand Junction Branch was replaced by a road bridge in 1908–09, followed by Webster Street in 1911.

[13] Numerous grade crossings were eliminated throughout the state in the 1930s; those on the Fitchburg Route included Boston Post Road (Route 20) at Stony Brook in 1930[14][15] and at Beaver Brook in 1936,[16] Mohawk Trail in Littleton in 1932,[17] and Leominster–Shirley Road in Lunenburg around 1936.

[21] Service west of Greenfield ended on December 30, 1958; stops dropped at that time were Williamstown, North Adams, Hoosac Tunnel, Zoar, Charlemont, and Shelburne Falls.

[22][21] On June 14, 1959, seven stops between Greenfield and Fitchburg (Montague, Lake Pleasant, Erving, Royalston, Baldwinville, East Gardner, and South Ashburnham) plus Stony Brook were dropped as part of another round of systemwide cuts.

[23][24] The four daily round trips west of Fitchburg were discontinued on April 23, 1960, ending service to Greenfield, Millers Falls, Athol, Orange, and Gardner.

[21][25][26][27] On January 18, 1965, service was cut back to West Concord, but was restored to Ayer on June 28, 1965.

The Fitchburg Line west of the old Stony Brook Railroad, which now junctions east of the old Ayer Junction, now serves as part of Pan Am Railways' main line between Mattawamkeag, Maine, and Mechanicville, New York.

Freight operation ended in 1981, and the line was formally abandoned in 1991 to make way for the Minuteman Commuter Bike Trail.

[28]: 137 The Watertown Branch Railroad was incorporated 1847, first as an independent short line RR, but was quickly taken over by the Fitchburg.

The entire branch is now either abandoned or out of service, and the east side of the right-of-way was converted to a rail trail - the Watertown-Cambridge Greenway.

This branch made South Acton a major junction and service point on the Fitchburg Route.

A turntable and engine house existed in South Acton to service trains well into the 20th century.

Passenger service from Marlborough ceased in 1932, and the section between Maynard and Hudson was abandoned in 1943.

The Peterborough and Shirley Railroad was incorporated in 1845 and opened as a branch from the Fitchburg in Ayer to West Townsend in 1848, continuing to Mason, New Hampshire, in 1849 or 1850.

In July 2020, the state awarded $100,000 for removing tracks and paving the western third of the trail.

This was a continuation of the New London Northern Railroad, built south from Miller's Falls in 1867 and also leased to the Vermont Central in 1871.

Passenger service ended in 1958, and the line was abandoned in sections, Winchendon north in 1970 (after the bankruptcy of the Rutland RR) and in 1984 for the rest.

The first section opened from the state line to the west end of the tunnel at North Adams in 1859.

It opened in 1852, continuing as the Western Vermont Railroad (leased by the Troy and Boston from 1857 until it was reorganized into the Bennington and Rutland Railway in 1865).

The Boston, Hoosac Tunnel and Western Railway opened in 1879 between the Massachusetts state line and Mechanicville, New York.