M15 or M-15 may refer to:
The Second Avenue Line is a bus line in Manhattan, New York City, United States, running mostly along Second Avenue (and northbound on First Avenue since 1951) from Lower Manhattan to East Harlem. Originally a streetcar line, it is now the southbound direction of the M15 bus route, the busiest bus route in the U.S. with an annual ridership just shy of 17.5 million. (However, the M15 is the only route on First and Second Avenues, while the one-way pair of Madison and Fifth Avenues has a higher frequency of local buses past Central Park, split among four routes (M1/M2/M3/M4).MTA Regional Bus Operations, under the New York City Bus and Select Bus Service brands, operates the local out of the Tuskegee Airmen Bus Depot and the SBS is operated from the Mother Clara Hale Bus Depot. Service is operated exclusively with articulated buses.
The Second Avenue Railroad opened the line in 1853 and 1854, from Peck Slip on the East River north along Pearl Street, Bowery (shared with the Third Avenue Line), Grand Street, Chrystie Street, and Second Avenue to East Harlem. A short branch was later built along Stuyvesant Street and Astor Place to end at Broadway in NoHo. The Metropolitan Street Railway leased the line in January 1898, and on April 3 the line from Astor Place to Manhattan was electrified. The original line was later electrified to the Bowery, where streetcars used the Third Avenue Line to City Hall, and the line to Peck Slip was abandoned. Buses were substituted for streetcars by the East Side Omnibus Corporation on June 25, 1933. The New York City Board of Transportation took over operations in 1948, with the New York City Transit Authority replacing it in 1953. In 1974, the M15 was chosen to be one of the first routes to operate Limited stop service. The new service would act as an express version of the M15, only stopping at major attractions and transfer points north of Houston Street.
The M15 motorway is a Hungarian motorway connects the M1 motorway to Bratislava, the capital of Slovakia. The Hungary-Slovakia border crossing is at Rajka (Hungary) and Čunovo (Slovakia).
The M15 motorway is a somewhat peculiar motorway since it does feature grade separation like all motorways, but does not feature four lanes. It is planned that by 2015 the M15 will be upgraded to four lanes.