Mexico City Metro Line 2, the blue line, runs from northwest to east and then south. It's a big "7" shape, as the map shows.
The line is significant because it follows two of the "calzadas," i.e.; causeways that crossed the lake into ancient Mexico-Tenochtitlan. The Calzada Mexico-Tacuba ran to the west. The Calzada de Tlalpan, to the south.
The line began service in 1970 with service from Tasqueña to Pino Suárez. One month later, it was extended west to Tacuba. Finally in 1984, service was extended by two stations from Tacuba to Cuatro Caminos.
For international visitors, Line 2 is especially important for the stations in the City center. But riding from out from the center on Mexico City Metro Line 2 means you're traveling into some of the City's deepest history.