Here’s something interesting – an early concept diagram for the Helsinki Metro that shows two lines instead of the eventual one (plus branch). It appears in the 1974 edition of Graphis Diagrams, a collection of the best informational design from around the world at the time (generally within the last year). 

Stylistically, it’s almost identical to the diagram that was actually used when the Metro finally opened in 1982… it just has a whole extra line.

Graphis credits the designers as Ola Laiho and Esko Miettinen.

Source: typo et design website (JPG)