"伦敦的地铁系统" can be translated into English as "London's underground/subway system" or more formally, "the London Underground (often referred to as 'the Tube')".
"Underground" and "subway" are both commonly used terms to describe a subterranean railway system.
"The London Underground" is the official and most widely recognized name for London's metro system, with "the Tube" being a colloquial and affectionate nickname.
So, depending on the context and level of formality, you could use any of these expressions.