Information Design Aspects of the London Underground Diagram
It's called the Underground Map. This is, of course, not only quite wrong (it is not a map, it is a diagram) but the whole point is that it has shed its geographical features because a diagram is considered easier to use.
The London Underground Diagram was never designed. It is the product both of a small number of major design changes imposed upon once-indifferent mapping origins, and a constant stream of evolutionary changes not all of which have been successful (many – but not all – of the less successful changes have been discarded, and perhaps some of the better ones too). The lack of clear design criteria for the diagram may be responsible for otherwise inexplicable changes, especially with modern purchasing practice working against consistency of input by the design houses that are physically responsible for the changes made.
The author suggests that there are several developmental stages, each of which is set out. This starts off with the need to show lines that interchange, then clarify the distinction between different lines, then to codify the symbols so that the design hangs together as a whole, then finally to reorganize the lines in the form of a diagram.