Simplify the following text for a Grade 9 native English-speaking person.
Original: The tube map is the schematic diagram ( transit map ) representing the lines and stations of some of London 's rapid transit rail systems , the London Underground ( commonly known as the tube , hence the name ) , the Docklands Light Railway ( DLR ) and London Overground .
Simplified: The tube map is the diagram which represents the London Underground , the rail system of London .