Just for interest, this pre-war tube map shows various Underground extensions which never came about.
Notably it shows the “Northern Heights” developments of the Northern Line including a line from Mill Hill East via Edgware to Bushey. This line did not come about due to the post-war Green Belt – this meant that this extension would not finally be viable for the necessary extra passengers covering construction costs.
With the lack of progress here, the Alexandra Palace and Finsbury Park routes lost potential traffic and the City Line of the Northern Line via Drayton Park was cut off into an eventually nonviable stump of a line. The Northern Line is a very different animal today.
Also of interest is a Central Line extension to Denham from West Ruislip. Indeed all of this line beyond North Acton is unbuilt. In the east, none of the Central Line beyond Mile End is complete.
The Metropolitan Line, coloured green, doubles the Bakerloo on its Stanmore branch.
The District Line parallels to Piccadilly to Hounslow. Eastwards, the District is labelled as travelling as far as Southend.
Various stations have their former names, such as Mark Lane for Tower Hill.
