The London Underground is a specific entity which consists of 272 stations across eleven lines.
I recent years TfL have take to publishing maps which show most of London's local railways. Something like this:
https://tfl.gov.uk/maps/track?intcmp=40400
These maps show the Underground. Tfl refers to this as "Tube" This is a little unhelpful because very little of London Underground is actually in tube-sized tunnels and in fact only about 45% of it is under the ground at all. But they also show the DLR, the Overground, National Rail, the Elizabeth Line (which is not part of the London Underground or Overground) and the tram network.
Quizzes and puzzles which mention "The Underground" or "Tube" should make it clear what they are referring to.