Originally, it used the south tracks of the Manhattan Bridge, which at that time connected to the BMT Nassau Street Line. Trains began running express between Pacific Street and 59th Street with the extension of the Fourth Avenue Line to 86th Street. On September 4, 1917, the first part of the BMT Broadway Line and the north side tracks of the Manhattan Bridge opened. Trains ran from 14th Street–Union Square to Coney Island–Stillwell Avenue, now using the bridge's northern tracks. On January 15, 1918, service was extended to Times Square–42nd Street. Weekday and Saturday summer trains leaving Coney Island between 6:37 a.m. and 8:37 p.m. began running non-stop between Kings Highway and 59th Street on August 1, 1920. Trains that started at Kings Highway made local stops. On November 14, 1920, weekday rush hour trains, and Saturday AM rush hour trains began skipping Myrtle Avenue and DeKalb Avenue. Between 1924 and 1926, trains skipped Myrtle Avenue and DeKalb Avenue during weekday rush hours, and on Saturday southbound between 12:01 p.m. and 6:57 p.m. Between 4:30 p.m. and 7:30 p.m. weekdays, and 12:01 p.m. to 6:57 p.m. Southbound on Saturdays, the rear two cars at Canal Street were restricted to boarding only (no exiting the train). In June 1931, trains skipped Myrtle Avenue and DeKalb Avenue between 7 a.m. and 12:30 a.m. weekdays, and between 10:30 a.m. Sunday to 12:30 a.m. Monday. Trains skipped 36th Street during the morning rush hour in the northbound direction. Between 1933 and 1937, the hours in which trains skipped Myrtle Avenue and DeKalb Avenue were 7 a.m. and 12:00 a.m. weekdays. On June 29, 1950, trains began running express in Brooklyn during late nights. As of June 28, 1951, every other morning rush hour train no longer terminated at Kings Highway. On May 2, 1957, service was extended north via the express tracks from Times Square to 57th Street–Seventh Avenue, and trains stopped bypassing 36th Street during the AM rush hour. In fall 1959, trains began stopping at DeKalb Avenue during midday hours. Previously, they bypassed DeKalb Avenue at all times except late nights. Beginning on January 1, 1961, trains bypassed DeKalb Avenue during rush hours only. In addition, on weekday and Saturday evenings, late nights, and all day Sundays, they ran local on the BMT Fourth Avenue Line. The N designation began to appear when R27 subway cars were moved to the service in