tracks of the Joralemon Street Tunnel from Brooklyn, which become the express tracks. These run north under Broadway and Park Row to Centre Street. At the south end of Centre Street, directly under New York City Hall, is the City Hall Loop and its abandoned station, which was the southern terminus of the original IRT subway line. The loop is still used to turn 6 and <6> service; the Lexington Avenue local tracks, which feed the loop, rise up to join the express tracks just south of Brooklyn Bridge–City Hall station. From Brooklyn Bridge, the line continues northward in a four-across track layout under Centre Street, Lafayette Street, Fourth Avenue, and Park Avenue South until 42nd Street. At this point, the beginning of Metro-North Railroad's Park Avenue tunnel in Grand Central Terminal forces the Lexington Avenue Line to shift slightly eastward to Lexington Avenue; its Grand Central–42nd Street station is located on the diagonal between Park and Lexington. Just south of Grand Central, a single non-revenue track connects the IRT 42nd Street Shuttle to the southbound local track; this was part of the original IRT subway alignment. Under Lexington Avenue, the line assumes a two-over-two track configuration, with the local tracks running on the upper level and the express on the lower, although it briefly returns to a four-across layout between 96th Street and 116th Street. 125th Street returns to this two-over-two layout, although here the upper level is used by all northbound trains and the lower level by southbound trains. This is because Lexington Avenue is too narrow to have a four-across layout. North of this, the line crosses under the Harlem River into the Bronx via the four-track Lexington Avenue Tunnel, where the line splits into the IRT Jerome Avenue Line on the western two tracks () and the IRT Pelham Line on the eastern two tracks (). History Original subway Construction started on the first IRT line in 1900. A 1902 explosion during construction seriously damaged properties just above the line. The part of the line from City Hall to just south of 42nd Street was part of the original IRT line, opened on October 27, 1904. A extension to Fulton Street opened at 12:01 a.m. on January 16, 1905. Only the northbound platform opened at this time. The next station, Wall Street, was opened on June 12, 1905 as well as the southbound platform at Fulton