In Boston, subway and surface lines are identified by color: red, blue, green and orange. Recently, an express bus system was implemented, known as the silver line. Lines can be identified by color there because trains/streetcars are assigned to specific trackage. In NYC, the system is so large and complicated that trains can be rerouted from one line to another, so even though different bullet colors are assigned to specific routes, trains can often use two separately built lines on their routes. Out of towners often make the mistake of asking for the “green line” if they mean the 4, 5, 6 or the “yellow line” if they mean the N or the R.
In the 1970s, the MBTA — or the T as it’s known — developed a system signage scheme that incorporated line colors, a line strip map, a systemwide map and a street/topographical map, as well as words in the easily readable Helvetica Bold font. This combination appears in most stations.