There are 150 ‘metro’ systems in the cities of the world. Which is the longest is open to debate because there are several factors involved and it is not a simple calculation to make. Some have sections that run above ground and it is problematic to decide which elements of the individual systems should be taken into account. London’s tube is certainly among the longest, however, some of the newer Chinese metros are rapidly catching up or might already exceed its length.
Moscow’s metro is particularly notable for the impressive ornamentation of the stations, while New York’s is said to be the dirtiest.