The <6> Diamond 6 identifies the New York City Subway’s rush-hour express trips to the Bronx.
The 6 service normally operates as a local between Pelham Bay Park in the Bronx and Brooklyn Bridge–City Hall in Manhattan. During weekday rush hours in the peak direction, selected trains run express in the Bronx and are marked with a diamond-shaped 6 bullet.
Those express trains skip some local stations between Parkchester and Third Avenue–138th Street, reducing travel time for riders traveling toward the busiest parts of Manhattan. The regular local service uses a round green 6 bullet.
The diamond notation is not a separate numbered route in the same sense as an entirely different service. It identifies a special pattern of the 6 service. This differs from the <7> designation, which marks express operation on the Flushing Line.