The original plan called for the main line to turn west onto 125th Street with tail tracks to Fifth Avenue, while tail tracks would continue north on a spur via Second Avenue to 129th Street. However, the tail tracks to 129th Street, as well as a proposed ancillary building at 127th Street and Second Avenue, were removed in a June 2018 update to the plans. The change in tail tracks was made because it was found that providing tail tracks at the line's terminal will more efficiently facilitate subway service. A bellmouth provision for extension to the Bronx remains, though shifted closer to the 116th Street station at 118th Street. Here, two outer tracks will head west toward 125th Street while space for two inner tracks will allow for an extension to the Bronx. North of 120th Street, the line will be constructed through the use of TBMs. South of 120th Street, the line will utilize the 99th–105th and 110th–120th Streets tunnel sections built during the 1970s, with a cut-and-cover tunnel connecting the segments between 105th and 110th Streets.