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