There are numerous ways to distinguish transit lines, including using unique names or symbols. Here and many places elsewhere, our subway goes with colors. That's because colors are a simple and user-friendly way to organize a system.
Transit lines can have unique names, such as the Piccadilly or Victoria lines in London; numbers, like the Paris Metro; letters, like the San Francisco Muni; and many other designations. American transit systems often have the peculiar tendency to take the names of colors.
The primary reason to name transit lines after their colors on maps is simplicity: if I tell you to find the Silver or Green line on a map, it is quite easy. Finding the Wiehle-Reston East-Largo Line or Branch Avenue - Greenbelt Lines, as you would do if you used the end-stations for naming lines (like the San Francisco BART does), is more difficult; you would have find the all of the end stations first to identify the line.
Boston’s MBTA, commonly known as the “T," is a perfect example of color-based lines – in the 1960s, the system map adopted colors to make the T easier to navigate. Line colors represented different aspects of the city that they served, such as the Red Line serving Harvard, home of the Crimson sports teams.
Other systems, such as the Chicago 'L', adopted a color system to simplify and modernize its line names, as well as mirror other systems with color names such as Washington and Boston. Of the transit operators that the Federal Transit Administration has identified as heavy rail, light rail, commuter rail, or streetcar systems, 18 use colors for naming.
Primary colors are the most common line designations in the United States, with blue placing first, and green and red tying for second. Orange is also quite common, but the Washington Metro’s other two colored lines, Yellow and Silver, are not. The Chicago 'L' is the only major system to have rail names with a Pink and Brown Line.
This graph only includes major heavy rail (such as a metro or subway), light rail, commuter rail, streetcar, and bus rapid-transit lines in the United States. From Montreal, Quebec to Casper, Wyoming, there are countless other examples where transit systems use colors for nomenclature.
Most transit maps use colors, even if they are not included in the names. San Francisco’s BART system, for instance, uses blue, green, orange, red, and yellow to mark lines on its map, but the lines themselves go by the name of their final stations, such as the Daly City – Dublin/Pleasanton line.
The New York Subway is even more complicated, using letters or numbers to denote the service route (similar to what the Washington Metro refers to as a "line"), colors to denote services that run along the same route in the core, and shapes to denote if the service is express or local. So the number 4, 5, and 6 trains all use a green color since they all run along the same route in Manhattan, and number 6 trains additionally use a circle shape for local services and a diamond shape for express trains.
Color lines do have some disadvantages. Line colors can be confusing to people who are not familiar with the system. For instance, both the Blue and Silver Line go to Largo, but a tourist or new resident who does not understand that they can take either line may wait longer for a specific train, delaying them.
Using line colors in New York would also be impractical, as there are simply too many lines. After eight or so lines, using colors for each different route becomes impractical as there are no longer primary or secondary colors left to use for names. The New York Subway already use a darker green for the 4, 5, and 6 trains, and a lighter green for the G trains, a darker blue for the A, C, and E trains, a lighter blue for the T trains, and so on.
Using colors to identify all of these lines would not make the system simpler; it makes it more confusing. That's precisely the case with the bus system in Auckland, New Zealand, which has far too many colors.
What else have you noticed about how transit route names affect the ways people use them?