and each of the shuttle tracks in operation at any given time is independent of the other. Its route bullet is colored on route signs, station signs, and rolling stock with the letter "S" on the official subway map. History Creation and early years The subway through which the shuttle runs was opened on October 27, 1904, by the Interborough Rapid Transit Company (IRT), the first day of subway service in Manhattan. The current shuttle line was part of the first IRT subway line, which ran north to 145th Street via Broadway and south to City Hall via Park Avenue and Lafayette Street. The 42nd Street section of the line connected Broadway at Times Square, on the west, to Park Avenue at Grand Central Terminal, on the east. At the Times Square end of this segment, the line curved sharply to the north under One Times Square, swinging northeast under Seventh Avenue before shifting under Broadway. The platforms at Times Square are located on this curve. Like the rest of the Original Subway, the line was built with a vertical clearance of , and a total width of . The maximum grade of the line is 1.0 percent between Sixth Avenue and Broadway. In 1910, the platforms at the two stations were extended. In 1913, the IRT, the Brooklyn Rapid Transit, and the city agreed to the Dual System of Rapid Transit to expand the city's transportation. As part of the agreement, the existing IRT subway would be split into two north–south lines and a shuttle along 42nd Street. The section of the line south of 42nd Street would be connected to a newly constructed line stretching from 41st Street and Park Avenue to the Bronx, running via Lexington Avenue, while the section of the line north of 42nd Street would be connected to a newly constructed line heading south under Seventh Avenue. The section along 42nd Street was left as a shuttle to connect the new East Side and West Side Lines. The new Lexington Avenue route curved off of the old line at 41st Street and ran underneath private property to reach Lexington Avenue at 43rd Street with a new Grand Central station located in the diagonal segment. Since there was 400 feet between the eastern end of the original line's station and the new Lexington Avenue Line station, a new shuttle station was to be built near