station was opened on July 29, 1914, allowing for a second service, the daytime Myrtle Avenue–Chambers Street Line, or Myrtle-Chambers Line, which ran along the Broadway elevated and the Williamsburg Bridge to Chambers Street on the Nassau Street Loop in Lower Manhattan. Following the completion of a third track along the Broadway Elevated between Marcy Avenue and Myrtle Avenue on January 17, 1916, these trains began running express on the Broadway Elevated during the evening rush hour in the peak-direction. By 1920, trains later began running express in the morning rush hour and on Saturday afternoon in the peak direction. The number 10 was assigned to the service in 1924. At the time, service ran on weekdays between 6 a.m. and 8 p.m., on Saturdays from 6 a.m. and 9 p.m., and on Sundays from 12:30 to 11 p.m. In the morning rush hour, trains ran express between Central Avenue and Essex Street, and during the evening rush hour, trains ran express between Bowery and Broadway–Myrtle Avenue. Sunday service was removed in June 1933. All Saturday trains began running local on June 28, 1952. On February 10, 1958, the four rush hour Brighton-Nassau special trains began stopping at Neck Road and Avenue U. In addition, the evening rush hour trains began stopping at DeKalb Avenue, as had been done by morning rush hour trains. On June 28, 1958, Saturday service was discontinued. On May 26, 1959, midday service was eliminated, making the Myrtle-Chambers Line rush-hours only. Service had previously operated on weekdays from 6 a.m. to 8 p.m. Beginning on February 23, 1960, express trains began stopping at Marcy Avenue, which was originally a local stop. 1961–1978 In 1961, with the arrival of new subway cars which featured rollsigns with new lettered designations for the BMT's numbered services, the 10 was renamed the M. Since these cars were not assigned to the route, it remained signed as 10. However, the rush hour Nassau Street specials on the BMT Brighton Line and BMT Fourth Avenue Line were signed using the M designation. The line was officially designated "M" after the Chrystie Street changeover on November 27, 1967. The second half of the Chrystie Street Connection opened on July 1, 1968, and the , which had run along Nassau Street to Broad Street, was relocated through the new connection to the IND Sixth Avenue Line (and renamed the KK). To augment service