[1] The NSRL would connect Amtrak and MBTA Commuter Rail lines that terminate either at North or South Station.

[3] 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.

From 1901 to 1938, the Atlantic Avenue Elevated provided direct passenger service past North and South Stations.

The North–South Rail Link is proposed to fill all these awkward gaps in service, with direct connections requiring no transfers.

In June 2018, MassDOT released a North South Rail Link Feasibility Reassessment presentation that described several tunnel alternatives.

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.

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.

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.

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.

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.

[1] The proposal makes aggressive assumptions about improved passenger loading and unloading times and on how many trains can operate per hour in the tunnels.

[5] A North-South Rail Link was included in the original Big Dig design, but was dropped by the Dukakis Administration.

It would dig two 41-foot-diameter (12 m) deep-bore tunnels beneath Boston, extending beyond the present rail yards north and south of the city.

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.

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.

[19] 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.

[20] 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".

[22] In June 2018, MassDOT released a North South Rail Link Feasibility Reassessment presentation that proposed several tunnel alternatives, described above.

[1] The state began soliciting bids for the $2 million feasibility study, advocated for by the North-South Rail Link Working Group, in March 2017.

[9] On September 6, 2018, MassDOT released their draft feasibility reassessment report and asked for public comment by October 19.

[23] As of 2018[update], 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.

[24]: 12  The line would potentially be served by a new West Station which is proposed as part of the Beacon Park Yard redevelopment.

[25] 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.