Union Square station (Somerville)

New (left) and old stations in December 1911

The Charlestown Branch Railroad was extended through Somerville in 1841, opening on January 5, 1842. It was absorbed into the Fitchburg Railroad in 1846.[5] Prospect Street station – which was located off Medford Street (now Webster Street), slightly west of Prospect Street – was opened within several years to serve the Sand Pit Square area.[6][7][8] The nearby square became Union Square in the 1860s, but the station was not renamed Union Square until around 1875.[9]

Prospect Street was placed on a bridge over the railroad by 1852, but Webster Street crossed the railroad at grade.[10] Planning to eliminate the eleven remaining grade crossings in Somerville, five of which were on the Fitchburg Route mainline, began in 1900.[11] 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.[12]

After the Somerville Avenue grade crossing was eliminated in 1908–09, work began in April 1911 to raise Webster Street above the tracks.[13][14] Webster Street and several adjacent buildings were raised about 20 feet (6.1 m), while the tracks were slightly lowered. A special bridge on the east side of the road bridge was built for a 4-foot (1.2 m)-diameter water main that weighed 700 pounds per foot (1,000 kg/m). As part of the project, the 67-year-old station building was replaced by a larger building on the south side of the tracks.[8] Work was completed in December 1911.[15]

Horsecar and later electric streetcar service cut ridership at urban stations; by 1917, Union Square and Somerville stations were served by four to five daily Watertown Branch trains plus several off-peak mainline local trains.[16] As passengers volumes dwindled, the station building was reused for manufacturing by 1924.[17] By 1929, the only service to Union Square and Somerville was one outbound and two inbound Watertown Branch trains.[18] Service to the two stations ended on July 9, 1938, along with the end of passenger service on the Watertown Branch.[19][20][21] The station building was demolished sometime between 1936 and 1940.[22][23]

Adding an MBTA Commuter Rail Fitchburg Line station at Union Square has been considered; by 2003, a stop was expected to open within five years.[24] The 2004 state Program for Mass Transportation estimated such a station would attract 390 daily riders.[19] A Union Square commuter rail station – either in addition to a Green Line station or in lieu of it – was listed as a possibility in 2012 as an interim air quality mitigation measure in response to delays in building the Green Line Extension. However, such a station would have been costly to build and could not have been completed by the 2015 deadline, and was therefore not supported by MassDOT.[25]

Station construction in November 2020

In August 2012, the City of Somerville, MassDOT, and the MBTA reached a memorandum of agreement about the station. Through the Somerville Redevelopment Authority, the City will acquire $8 million worth of land for the station and grant the MBTA a permanent easement, while retaining the rights for transit-oriented development overhead. In return, the MBTA and MassDOT would pay for cleanup costs at the site, begin construction by the spring of 2014, and open the station no later than "late 2016-early 2017".[26]

In October 2012, the Somerville Board of Aldermen approved the Union Square Redevelopment Plan and authorized an $8 million bond, including $6 million to purchase the land and $2 million for cleanup and station planning.[27] In May 2013, the Board of Aldermen announced that the North Prospect block - a mostly industrial area bordered by the railroad tracks, Prospect Street, Somerville Avenue, and the rear of residential properties on Allen Street - had been acquired by the city via eminent domain for $4.5 million. The properties were to be vacated by August. The city also received a $1 million EPA grant to clean up one of the properties.[28] Site cleanup began in September 2014.[29]

After major cost increases due to contractors increasing costs above estimates became public in 2015, MassDOT cancelled contracts and scaled back sections of the project. A new design released in May 2016 removed direct access from the Prospect Street overpass (which required elevators, escalators, and stairs) and platform canopies.[30] In December 2016, the MBTA announced a new planned opening date of 2021 for the extension.[31] As of November 2019, a private developer will fund an elevator connecting the station to the overpass. The elevator may open later than the station.[32]

The station platform was poured by early August 2020, with the canopy constructed later that month.[33]

Original plans called for the E Branch to be extended to Union Square.[34][35] However, in April 2021, the MBTA indicated that Union Square would instead be served by the D Branch.[36] By March 2021, the station was expected to open in October 2021.[37] This was delayed in June 2021 to a December 2021 opening, and in October 2021 to a March 2022 opening.[38][39]

Extensive new transit oriented developments are planned surrounding the station, beginning with a 25-story residential tower and mid-rise commercial building, known as D2.3 and D2.2.[40]