Measuring the most extensive metro system in the world isn’t as cut and dry as it seems. First off, what exactly constitutes a metro? Does it have to be underground? And do we count the number of stations or the length of the track? Generally the two follow one another closely but not always. For the purposes of this list we decided to go with length of track but we are by no means claiming that this is the best or only way. So keeping all of this in mind we bring you the 25 most extensive metro systems in the world.