A freedom of information request sent to Transport for London in 2013 turned up this 2009 map of the London Underground’s track network (17.1 MB PDF)—complete with sidings, switches and yards. Among other things, you can see how a train can cross from one line to another. CityMetric picked up the story last week and it’s gone seriously viral since then: Boing Boing, Jalopnik, Wired.

If this is the sort of thing that fascinates you, you should go look at Franklin Jarrier’s maps of urban rail networks (which I told you about in 2011). These aren’t official maps, but they do for many systems around the world what the map above does for the Tube.