I love maps. In any history book, I always turn to the maps of how countries waxed and waned over the centuries. I love looking at transport maps - rail maps, tube maps, bus spider maps. One of my very favourite books is Transit Maps of the World, (sadly, it appears to be out of print) - a collection of historical and current tube/metro maps from around the world - with special attention paid to the largest metro systems, but with many hundreds of cities covered. I have whiled many an hour poring over this when I should be doing something else, sitting on the loo gawping at a map, when I should be vacating the bathroom, and generally looking at maps again and again. I have even designed an extra line - the Albert Line - for the tube.
You can imagine my delight on discovering this: http://www.londonreconnections.com/assets/london2050.svg It is a map laying out the TfL of 2050 if all the planned, proposed, and currently under construction lines are built. It comes from this page: http://www.londonreconnections.com/2014/london-2050-cartographical-interlude/
Watch, study, pore, and enjoy!
PS, and just look at this one, from the comments: http://projectmapping.co.uk/Reviews/Resources/London%20Rail%202018.pdf