This is a really cool map, showing the full geographically accurate (as far as I can tell) track layout of all railways and tramways in London. London Underground, London Overground and closed railways are all included, as is Tramlink in South London. Also included are the start and end of tunnels and the location and layout of depots and sidings.
Unfortunately, it doesn’t feature any old tram lines or anything that was proposed but abandoned, even if building work had started (e.g. the Northern Line extension to Bushey Heath). However, it does include lines currently under construction (e.g. Crossrail) and that have not yet been started (e.g. the Northern Line extension to Battersea).
From the map you can get a real sense of where the railway capacity is in London as well as where stations, minor branch lines and useful non-radial links have been lost. The additional diagrams of historic track and junction layouts are particularly interesting. I am surprised and impressed that someone managed to find all of this information!
There are maps for additional cities and regions available too, from a drop-down menu above the map. I’d like to see Dublin’s railways, Luas and DART on a similar map.