Have you ever stopped to consider which metro systems in the world are the most extensive? Measuring the most extensive metro system in the world isn’t as cut and dried as it seems. First off, what exactly constitutes a metro? Does it have to be underground? 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 to measure a metro system. Keeping all of this in mind, we bring you the 25 Most Extensive Metro Systems In The World.