Found here, of all places: http://www.sabre-roads.org.uk/forum/viewtopic.php?f=8&t=26696
A geographically accurate London Underground map; showing platforms, sidings, crossovers, opening dates etc etc.
Do you wonder where the actual tunnels go, instead of being satisfied that they run somehow between stations X and Y? Then this is for you.
There are also similar maps on that site of the Paris Metro, Barcelona and other places.