A legacy of the M train’s brown identification “bullet” can be found at an Amtrak underpass at 32nd Avenue and 56th Street in Woodside where, incidentally, no “subway” except the #7 Flushing Line El (identified by a purple bullet) runs.
No subway line has been shuffled around as much as the M — today it has two terminals in Queens, at 71st/Continental and Metropolitan Avenue, running on the Queens Boulevard, 6th Avenue and Broadway Brooklyn lines. However, the M used to run from Metropolitan Avenue to Coney Island, where it ran on the 4th Avenue subway and West End el. Before 1978, the M actually had a light blue ID bullet, but the colors were changed at the same time double letters (which usually denoted local lines) vanished from the nomenclature.
When the M was rerouted onto the 6th Avenue line, its bullets were switched to the orange used by other trains running on 6th: the B, D and F.
This leaves the J as the only “brown bullet” line remaining. Formerly, the R would run occasional rush-hour trains to Broad Street in lower Manhattan using the Nassau Street line where J trains ran; those trains carried a brown R diamond, at least in those cars using roll signs.