North–South Rail Link
The North–South Rail Link (NSRL) is a proposed rail tunnel, or pair of tunnels, that would connect North Station and South Station in downtown Boston, Massachusetts. The project would build new underground stations near the existing stations, connect them with about  of tunnels, and add other tunnels to link up with existing surface tracks. 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 June 2018, MassDOT's North South Rail Link Feasibility Reassessment described the costs and benefits of several alternatives, including adding tunnels in various configurations or doing nothing. The tunnel options included a $21.5 billion four-track maximum-service plan and three double-track routes ranging in cost from $12.3 billion to $14.7 billion. (These costs are in 2028 dollars and include purchasing additional rolling stock, other required infrastructure improvements, and a 3.5% annual inflation.) The presentation 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. Historical connections

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. Present connections

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 significantly to Orange Line congestion. Similarly, it is possible to connect between South Station and the Fitchburg Line via the Red Line at Porter station. Passengers on the Newburyport/Rockport Line can also take the Silver Line from Chelsea (Bellingham Square). The weekday-only MBTA bus #4 connects the two stations. The North–South Rail Link is proposed to fill all these awkward gaps in service, with direct connections requiring no transfers. 2018 NSRL feasibility study

In June 2018, MassDOT released a North South Rail Link Feasibility Reassessment presentation that described several tunnel alternatives. The state began soliciting bids for a $2 million feasibility study on March 1, 2017. This study had been pushed for by the North-South Rail Link Working Group. The working group, NSRL supporters, includes Senator Ed Markey, Representatives Niki Tsongas and Katherine Clark, with Representative Seth Moulton leading the effort. It also includes more than half of Massachusetts mayors. Options
The 2018 MassDOT feasibility study analyzed four possible options. Two options would run under the Central Artery (I-93), one option with two tracks and the other, more expensive, with four. The other two options, each with two tracks, would tunnel under downtown Boston several blocks west of the Artery. All the proposals connect the northern commuter rail lines with South Station and then to Back Bay and lines to the west. Only the four-track option would connect North Station with the Fairmount Line south of South Station. Under all the proposals, the Old Colony Lines would continue to terminate at the South Station surface platforms. The existing North Station and its surface platforms could be eliminated under all four options, making the area available for redevelopment. Central Artery four-track
The most ambitious option is the only one that includes a new Central station (or Union Station) near its midpoint, and the only one with a rail tunnel connection to the Fairmount Line. North Station would be underground near the present station. The line would be constructed using two  TBM tunnels,  deep, along with cut-and-cover in Fort Point Channel. Underground South Station would be built in Fort Point Channel, east of the surface station. Central station would connect to the Blue Line at Aquarium station. The Blue Line is currently the only MBTA rapid transit line with no direct connection to the commuter rail network. Estimated cost: $21.491 billion. Central Artery two-track
Same route as Central Artery four-track but has no Central station. Constructed using one  TBM tunnel,  deep, with cut-and-cover used in Fort Point Channel. Underground South Station would be built in Fort Point Channel, east of surface station. Estimated cost: $12.317 billion. South/Congress alignment
North Station would be entirely underground and moved several blocks south, to between State and Haymarket Streets; no Central station. Stacked tracks and platforms are incorporated within a single  TBM bored tunnel,  deep. Underground South Station would be north-west of surface station. Estimated cost: $13.181 billion. Pearl/Congress alignment
North Station would be entirely underground and moved several blocks south, to between State and Haymarket Streets; no Central station. Tracks would be in two  TBM bored tunnels,  deep. Underground South Station would be north of surface station. Estimated cost: $14.388 billion. Electrification
The tunnels would not be able to handle diesel locomotives and therefore would require the use of dual-mode locomotives or the electrification of several MBTA commuter rail lines. Full electrification of additional rail lines, proposed in earlier studies, could help Massachusetts and the MBTA reach their greenhouse-gas reduction goals, make service more reliable, allow trains to accelerate more quickly, and reduce travel times, but it is expensive and only limited electrification is included in the 2018 proposals. Part of the Fairmount Line would be electrified to allow Amtrak and MBTA Northeast Corridor trains currently routed through Back Bay to reach South Station during construction of the Back Bay portal. The electrified service region would also incorporate the proposed South Coast Rail project's Phase II, which includes electrification to Fall River and New Bedford. Benefits

The 2018 feasibility study modeled ridership in 2040 based on various scenarios. In regular service, the two-track options would increase all day boardings to 195,000 compared with 150,000 in the no-build scenario. Assuming an upgrade to all-day peak service,  the two-track options would increase all day boardings to 225,000 compared with 195,000 with the South Station expansion. Overall, NSRL would increase commuter rail capacity, in terms of seats to downtown, by 25%. The feasibility study presentation also projected the distribution of destinations for commuters coming from the north in the morning. Potential problems
The 2018 feasibility study identified several potential problems and risks with the NSRL project:

During construction of the Back Bay tunnel portal, required under all options, the MBTA Worcester Line and Amtrak Lake Shore Limited service would have to terminate west of Back Bay station or possibly be rerouted via the Grand Junction Line into North Station. Also some or all Amtrak, Providence, Stoughton, and Franklin Line service would be rerouted via the Fairmount Line into South Station. The proposal makes aggressive assumptions about improved passenger loading and unloading times and on how many trains can operate per hour in the tunnels. The proposed train frequency, 17 per hour per direction in the two-track options, is higher than the current Red Line train frequency, and exceeds what has been achieved to date on other systems that use the new positive train control systems required for heavy rail. To maintain the 17 trains/hour frequency, trains would have to arrive to enter the tunnel precisely as scheduled. However, new systems like Thameslink in London, UK, have shown that up to 24 trains per hour are possible on heavy rail. 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. While the NSRL design as proposed exceeds National Fire Protection Association NFPA 130 life safety requirements, evacuation of the deep tunnels and stations in case of fire could still be difficult. The deep stations would also cost passengers additional time entering and leaving. Currently Porter Square station, at 105 ft, is the deepest in the MBTA system. Earlier proposals
There have been several prior proposals to link South Station and North Station by rail. A North-South Rail Link was included in the original Big Dig design, but was dropped by the Dukakis Administration. The 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  deep-bore tunnels beneath Boston, extending beyond the present rail yards north and south of the city. The tunnels would pass about  beneath the I-90 extension, and would reach their maximum depth of  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  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  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  platform; North Station and South Station would have  platforms. Pilings for a planned high-rise 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  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. That would require the MBTA to buy and run locomotives equipped for electric operation. Route map
The map below depicts one of the most extensive proposals made to date. The 2018 feasibility study options do not include a link to the Old Colony and Greenbush lines. Only the four-track option includes a link to the Fairmont Line and a Central station. The surface North Station would be eliminated in two of the options and might not be kept in the other two. Status

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. The state began soliciting bids for the $2 million feasibility study, advocated for by the North-South Rail Link Working Group, in March 2017. The study did not identify a funding source for constructing the NSRL. On September 6, 2018, MassDOT released their draft feasibility reassessment report and asked for public comment by October 19. In late 2022, with the election of Maura Healey as governor of Massachusetts, advocates of the North-South Rail Link began pushing for support for the project. Alternatives
, 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. An above-ground rail link between South and North Stations was once proposed by the Boston-based Association for Public Transportation (which also supports the underground North–South Rail Link). This would eliminate the need to use multiple rapid transit lines to travel between terminals, but would still require two transfers for Amtrak and commuter rail passengers passing through downtown Boston. Unless grade separated, the link would have to operate at slow speed through very congested downtown traffic. References 
Proposed Monorail Would Link Boston's North, South Stations
North–South Rail Link too expensive, panel says
The conservation connection – The Boston Globe
William M. Fowler Jr., director of the Massachusetts Historical Society 
The MASSACHUSETTS SIERRA CLUB: North South Rail Link

Further reading

External links
North South Rail Link - an advocacy website

Amtrak tunnels
MBTA Commuter Rail
Passenger rail transportation in Massachusetts
Proposed railway lines in Massachusetts
Proposed railway tunnels in North America
Proposed transportation infrastructure in Massachusetts
Proposed tunnels in the United States
Rail infrastructure in Massachusetts
Transportation in New England
Underground commuter rail