Железнодорожный туннель между Северным и Южным вокзалами Бостона

Proposed rail tunnel connecting Boston's North and South Stations

The NSRL would connect Amtrak and MBTA Commuter Rail lines that terminate either at North or South Station. For MBTA, it would link northern lines to South Station, Back Bay Station, and lines beyond Back Bay, including the Framingham/Worcester Line and the Northeast Corridor; one option under consideration would also link to commuter lines to the south. The project would also link Amtrak's various trains into and out of the city; for example, the Downeaster line from Maine currently has no direct connection with Northeast Corridor trains to New York City and beyond. In 2017, a Harvard Kennedy School study estimated the cost of the NSRL at $4 to $6 billion (in 2025 dollars), based on Federal Transit Administration data and an analysis of comparable tunnel projects around the world. A 2018 MassDOT study looked at four options with estimated costs ranging from $12 billion to $22 billion (in 2028 dollars). As of , the North South Rail Link has not been pursued and is not in active development.

From 1872 to 1969, the freight only Union Freight Railroad provided a direct, street running connection between most of the south side and north side railroads, and served local customers and wharves in between. From 1901 to 1938, the Atlantic Avenue Elevated provided direct passenger service past North and South Stations. The elevated trackage was not connected to any of the conventional railroad tracks.

Public transit connects North Station to South Station only indirectly, through two of the MBTA Subway system's quartet of hub stations. A trip between the two commuter rail stations requires taking two subway lines, either the Green Line and the Red Line through their shared Park Street station, or the Orange Line and the Red Line via their shared Downtown Crossing station. The underground Winter Street Concourse connects the upper levels of Park Street and Downtown Crossing stations. Amtrak recommends that passengers with young children or much luggage take a taxi between the stations. It is possible to traverse the gap directly via the Orange Line from Back Bay Station to North Station, but it requires seven stops and not all of the southern lines pass through Back Bay; the Old Colony Lines, and Fairmount Line on the Commuter Rail do not. The Orange Line also provides a connection for Amtrak passengers who want to transfer between the Northeast Corridor and the Downeaster. However, North Station passengers add to Orange Line congestion.

The 2018 MassDOT feasibility study analyzed four options. The costs, presented in 2028 dollars, include purchasing additional rolling stock, other required infrastructure improvements, and a 3.5% annual inflation. Presenters noted that a planned expansion of South Station would cost an estimated $4.7 billion, not including the purchase of rolling stock, electrifying the system, and renovating Tower 1. Estimated cost: $21.491 billion. With the NSRL, the MBTA commuter rail system would be highly dependent on the tunnel routes. Any disruption to those routes would affect the entire system. An earlier leading proposal, called the "Dorchester Avenue Alignment", is similar to the four track Central Artery alignment in the 2018 study. It would dig two 41 feet (12 m) deep bore tunnels beneath Boston, extending beyond the present rail yards north and south of the city. The tunnels would pass about 20 feet (6 m) beneath the I 90 extension, and would reach their maximum depth of 130 feet (40 m) at a possible Central Station and at North Station. Carrying a total of up to four tracks, the tunnels would have steep inclines. Trains entering or exiting the tunnels would climb or descend 3% grades, each nearly 1 mile (2 km) long. Because the tunnels would continue well south of downtown, three portals would accommodate separate connections to Back Bay Station to the west, the Old Colony Lines to the south, and the Fairmount Line running southwest. To the north, the tunnels would cross the Charles River about 70 feet (21 m) below its surface (bypassing an existing drawbridge), and connect via portals to the Fitchburg Line and the other northbound rail lines. The plan would require two or three new underground stations; stations are proposed roughly beneath the current North and South Stations, and possibly a new Central Station near Aquarium Station. Central Station would have an 800 feet (244 m) platform; North Station and South Station would have 1050 feet (320 m) platforms. Pilings for a South Station Tower at South Station complicate a proposal to put the tunnels directly beneath the present South Station. Instead, the Dorchester Avenue Alignment proposal would move the tracks just east of South Station, and would build an underground facility about 100 feet (30 m) below the surface of the Fort Point Channel at the Summer Street crossing. Tracks at the underground South Station would have a 0.61% incline. The new Central Station would connect with the Blue Line at its Aquarium station via an underground concourse, as the Blue Line is the only rapid transit line in Boston that does not already connect with either North or South Station. The new station also would eliminate or reduce transfers to the rapid transit system for many commuter rail passengers with destinations in the central part of the business district. This would relieve transit congestion in the downtown core. Like Philadelphia's SEPTA system after the similar Center City Commuter Connection tunnel was built and connected two commuter rail systems, some of Boston's trains would be through routed from one side of the system to the other. Many services would still terminate at North and South Stations, on existing tracks that do not lead into the tunnels. This could also allow trains to pass parked train cars. A 2003 DEIR/MIS assumes that about one third of Amtrak trains to and from points south would be routed through the tunnel, stopping only at South Station, but with a new stop north of Boston at Anderson Regional Transportation Center in Woburn, Massachusetts. The Downeaster service from Maine and New Hampshire was assumed to stop at North Station only, with a direct connection to more southerly service at Anderson RTC rather than Boston. Thus, some operations would continue above ground at North Station and South Station, and all track and facilities would remain in place. The tunnels would not be equipped to handle diesel locomotives, which may not be suited to the planned steep grades and closely spaced stops; several Commuter Rail lines would necessitate electrification to safely and efficiently operate in the tunnels.

In May 2006, the Commonwealth of Massachusetts withdrew its sponsorship of the project due to its high capital cost (projected at several billion dollars, with wide variations depending on which option is chosen). Without matching local funds, the project was ineligible for federal funding, and was no longer listed as an approved project in state and Boston MPO capital plans. The April 2007 document Journey to 2030: Transportation Plan of the Boston Region Metropolitan Planning Organization said "the MPO feels that a study of the right of way requirements should be conducted for preservation of that right of way so as to not preclude this project's going forward in the future." In December 2007, the Federal Railroad Administration was interested in funding this project if the Massachusetts Executive Office of Transportation was interested in sponsoring it. In August 2009, the project was brought back into the spotlight as a component of the New England transportation plan, a coordinated effort by the six New England states to improve rail transportation infrastructure by competing for the $8 billion allocated for high speed rail in the American Recovery and Reinvestment Act of 2009. In December 2011, former Governor Michael S. Dukakis reiterated his long term support for rail service, saying he had been trying to convince the current Massachusetts administration "to get serious about building a rail link" rather than expand South Station. In January 2014, Dukakis said he would prefer to have the North–South Rail Link named after him, rather than South Station, as the Massachusetts House of Representatives had voted unanimously to do. In August 2015, former Governors Dukakis and William F. Weld co wrote an op ed calling the link "One of the most important and cost effective investments we can make". Based on their advocacy, MassDOT agreed to fund a $2 million study in February 2016. In June 2018, MassDOT released a North South Rail Link Feasibility Reassessment presentation that proposed several tunnel alternatives, described above.

as of 2018, the MBTA and Amtrak use the Grand Junction Line for non revenue vehicle moves between the two sides of their networks. This alternative connection splits from the Framingham/Worcester Line near Boston University and the Mass Turnpike Allston/Brighton exit ramps, and the track then crosses the Charles River into Cambridge. From there, it runs through the East Cambridge neighborhood and into Somerville, where it connects to the commuter rail lines running from North Station, just below the McGrath O'Brien Highway. The line is currently single tracked east of Massachusetts Avenue and slow, with a large number of at grade crossings. Several of the crossings (e. g. Massachusetts Avenue, several streets around Kendall Square, Cambridge Street, and Gore Street) require trains to come to a near complete stop before proceeding at their maximum allowed track speed of The line would potentially be served by a new West Station which is proposed as part of the Beacon Park Yard redevelopment. The Grand Junction corridor has also been proposed as part of the Urban Ring light rail or bus rapid transit project, or a possible pedestrian trail. However, only Worcester Line trains would be directly served, while all other trains from South Station lines would have to detour and reverse all the way to the west of Back Bay and Lansdowne stations to reach the Grand Junction connection.