to the east, while the southbound platform was extended to the east. On January 23, 1911, ten-car express trains began running on the East Side Line, and the next day, ten-car express trains began running on the West Side Line. Dual Contracts After the original IRT opened, the city began planning new lines. In 1913, as part of the Dual Contracts, the New York City Public Service Commission planned to split the original IRT system into three segments: two north-south lines, carrying through trains over the Lexington Avenue and Broadway–Seventh Avenue Lines, and a west-east shuttle under 42nd Street. This would form a roughly "H"-shaped system. The Broadway–Seventh Avenue Line would split into two branches south of Chambers Street, one of which would turn eastward through Lower Manhattan, run under the East River via a new Clark Street Tunnel, and connect with the existing Contract 2 IRT Brooklyn Line at Borough Hall. Construction of the Clark Street Tunnel began in 1914. The Lexington Avenue Line north of Grand Central–42nd Street opened on August 1, 1918, and all Eastern Parkway Line services were sent via the Lexington Avenue Line. Less than a year later, on April 15, 1919, the Broadway–Seventh Avenue Line station opened when the Brooklyn branch of that line was extended from Wall Street in Manhattan to Atlantic Avenue in Brooklyn. Through service between the Brooklyn Line and the Broadway–Seventh Avenue Line was inaugurated with this extension. In 1920, a project to construct a new entrance at the northwestern corner of Joralemon Street and Court Street was completed. That year, the Eastern Parkway Line was extended east of Atlantic Avenue. The Joralemon Street Tunnel services, which had previously served all stops on the Eastern Parkway Line, became express services, while the Broadway–Seventh Avenue Line services ran local on the Eastern Parkway Line. Also planned under the Dual Contracts was the Broadway Line and Fourth Avenue Line of the Brooklyn Rapid Transit Company (BRT), subsequently the Brooklyn–Manhattan Transit Corporation (BMT). Under the Dual Contracts, the Whitehall–Montague Street route and the Montague Street Tunnel were to be built, connecting the Broadway Line in Manhattan and the Fourth Avenue subway under the Flatbush Avenue Extension to the west of the DeKalb Avenue station. Construction of the tunnel began in 1914. Service via the Montague Tunnel began on August 1, 1920, along with the opening of the Court Street station. Later years The