The M train in the NYC Subway runs 24 hours a day and 7 days a week between Metropolitan Avenue in Queens and Myrtle Avenue in Brooklyn. Weekdays, it is extended via 6 Avenue and 53rd Street to Forest Hills 71-Continental Avenues. It was the result of the the M/V combination in which the V designation was eliminated and replaced by the M. Basically the M train was rerouted from the Nassau Street line and 4th Avenue in Brooklyn to 6 Avenue and Queens Boulevard to replace the V.
Here is the current M logo in orange (takes the color of the trunk line in Manhattan in this case 6 Av)
Former M logo is brown for Nassau Street.
The designation which was discontinued. The V did run to 2nd Avenue, no longer served after the M/V combo.
The M train using R-160 cars at Broadway Lafayette Street via 6 Avenue.
This service pattern while a cut, has a benefit too. Residents now have a direct ride from the BMT Myrtle Line to Midtown Manhattan.