If you’re going to use icons for each of your stations, as Mexico City does, then why not make them nice and big and simply arrange them in the correct order?

More from Wikipedia on the iconography of the Mexico City Metro:

Each station is identified by a minimalist logo related to the name of the station or the area around it. This is because, at the time of the first line’s opening, the illiteracy rate was extremely high, so people found it easier to guide themselves with a system based on colors and visual signs. The design of the icons and the typography are a creation of Lance Wyman, who also designed the logotype for the 1968 Summer Olympic Games at Mexico City. The logos are not assigned at random; rather, they are designated by considering the surrounding area, such as:

• The reference places that are located around the stations (e.g., the logo for Salto del Agua fountain depicts a fountain);
• The topology of an area (e.g., Coyoacán—in Nahuatl “place of coyotes”—depicts a coyote); and
• The history of the place (e.g., Juárez, named after President Benito Juárez, depicts his silhouette).

The logos’ background colors reflect those of the line the station serves. Stations serving two or more lines show the respective colors of each line in diagonal stripes, as in Salto del Agua.

Source: Universe’s universe/Flickr