(depending on other elements of the tunnel design), but add between $540 million and $817 million (about one-sixth) to the total costs of construction. During the renovation of Aquarium station, slurry walls were built to a depth of to futureproof for the construction of the NSRL and Central Station. A 2018 re-evaluation of the NSRL examined two-track and four-track alternatives under the Central Artery, as well as two-track alternatives under Congress Street (which was rejected in the 2003 report). Only the more-expensive four-track option would include Central Station. However, the Congress Street alternatives would have their northern station under Haymarket Square rather than under the existing North Station; this would allow for a Blue Line connection at State rather than Aquarium. References External links MBTA – Aquarium Google Maps Street View: Long Wharf headhouse, State Street headhouses Blue Line (MBTA) stations Railway stations located underground in Boston Railway stations in the United States opened in 1906 Railway stations closed in 1949 Railway stations in the United States opened in 1950 Railway stations closed in 2000 Railway stations in the United States opened in 2001