The N train is unique in that it is the only current line of the New York City subway that runs on exclusively BMT
The first portion of what is now the N route to be built was a mid 19th century railway in then rural Southern Brooklyn that took holiday makers to the resort at Coney Island. It was called the Sea Beach line. Eventually the Brooklyn Rapid Transit corporation, ancestor of the Brooklyn Manhattan Transit corporation, acquired the Sea Beach line, as it did all of the railways of Brooklyn.
After the BRT built its first subway line, the line that runs up Fourth Avenue in Brooklyn, under the East River, and under Broadway to midtown Manhattan, the Sea Beach line was connected to this line for through service. In the 20's dual contract era of subway expansion, the BMT and the Interboro Rapid Transit (IRT) were jointly given the contract to operate two lines running under the East River to Queens, the Flushing line (today's 7 Train, and the Astoria line to Ditmars Boulevard, over which the N currently runs on the Queens end).
The city took over the competing transit systems in 1940, and after letter designations were given to the now unified lines, the N was the route that ran over the Sea Beach line, through the Fourth Avenue and Broadway subways, and through the new 60th street tunnel to the Independent (IND) Queens Boulevard line. This first incarnation of the N was actually one of the first hybrid BMT/IND lines. At the time the RR (later the R) was the route that followed the Astoria line.
Later the terminals of the N and R were switched and the N assumed the form it has today, following the Sea Beach line from Coney Island to the Fourth Avenue subway, under the East River to the Broadway subway and northwest through Manhattan, turning east, back under the river through the 60th street tunnel and along the Astoria line. The N route features many of the sharp curves of the original BMT subway, and the Queens Plaza station that it shares with the 7 Train in the only station in which BMT/IND trackage and IRT trackage share the same platform, a legacy of the dual operation of the Queens lines.
The N Train runs mostly R68 rolling stock, disgusting 75' cars with a mirrored interior from the early 80s. It also occasionally features an R40 train.