- New York City Subway
Considered to be the largest subway system in the world, the New York City Subway was established on October 27, 1904. The station is one of very few that is open throughout the entirety of the day and being that is located in the largest city in the US the station sees 1.7 billion passengers per year use it as transportation.
- Paris Métro
The Paris Métro is known for its artistic features seen throughout the entrances and exits. The Métro opened on July 19, 1900, is located in Paris, France and station has 1.5 billion passengers per year.
- Glasgow Subway
The Glasgow Subway debuted on December 14, 1896, the station covers a small region and is one of only two underground transportation systems in the UK. The Glasgow Subway is located in Scotland and only carries just under 40,000 passengers a year with 39,980.
- Budapest Metro
Opened in 1896, the Budapest Metro holds the honour of having the first underground electric train on the European Continent. The original line is still in use, 409 million passengers board the metro each year.
- London Underground
The first of subway transit began here. Opening on January 10, 1863. What is interesting about this subway station is that most of the tracks are found and run above ground. When the tunnels were first created they were used as walkways for pedestrians and later transitioned into tunnels for trains. 1.3 billion commute on the London Underground every year.
*You can find the original source of this information at: http://www.oldest.org/geography/subway-systems/