The last line to open was the Jubilee Line, in 1979. People tend to regard the Docklands Light Railway as part of the Tube network, although it doesn’t seem to be, at least officially; it opened in 1987. Although it seems unlikely that new lines are going to be built, at least in the foreseeable future, existing lines are sometimes added to. The Jubilee Line was famously extended in 1999, having been rushed in order to be open in time for the millennium. There are currently plans to extend the East London Line further north, although I’m not sure when the work will be completed.