The M15 bus, which runs along much of the IRT Second Avenue Elevated Line's route, carries more passengers than any other route in New York City. However, it does not carry as many passengers as a rapid transit line, and does not allow for interchange within rapid transit stations. A replacement rapid transit route, the Second Avenue Subway has been under consideration since before the demolition of the IRT Second Avenue Line, and is under construction as of 2007. The first phase is scheduled to open in 2014.