This is a map of the London Underground showing exactly when every line (and segment of line) and every station was opened and any subsequent closures, as well as all the changes in station names. There are a few odd explanatory paragraphs, but most of the information lies in the map. I found this presentation excellent as it provided, quite literally, a comprehensive picture of the odd pieces of information I had gleaned from other sources. My 6-year-old son particularly enjoyed tracing the tracks by the years and seeing how the undergroud had formed (and disappeared !) over the years. Overall, I found this a very good introduction to the London Underground, although it may be best appreciated in conjunction with a historical account (we found John Glover's "London's Underground" an excellent companion book).