Train services on the New York City subway carry a white number or letter in a colored circle or diamond, called a bullet, identifying the route that the train service takes along the various lines.
Those lines are grouped by color denoting that at some point on their route they share a common section of track, but they do not necessarily have the same start and/or end points. Accordingly, trains should always be identified by their letter or number rather than line color, as trains can have different stops and ultimate destination even if they share the same line color.
Local trains stop at every stop on a route, while Express trains skip some stops. When the service number is shown in a diamond, as is the case with some 6 and 7 trains, the service can be either express or local in certain sections, depending on the time and direction of travel.