What is the underground railway system in Paris called?

The Paris Metro or Métropolitain is a rapid transit system in the Paris Metropolitan Area. A symbol of the city, it is noted for its density within the city limits and its uniform architecture, influenced by Art Nouveau. It is mostly underground and 214 kilometres (133 mi) long. It has 303 stations, of which 62 have transfers between lines. There are 16 lines, numbered 1 to 14 with two lines, 3bis and 7bis, which are named because they started out as branches of lines 3 and 7; later they officially became separate lines. The Metro is still numbered as if these lines were absent. Lines are identified on maps by number and colour, and direction of travel is indicated by the terminus.

It is the second busiest subway system in Europe, after the Moscow Metro, and the tenth-busiest in the world. It carried 1.520 billion passengers in 2015, 4.16 million passengers a day, which amounts to 20% of the overall traffic in Paris. It is one of the densest metro systems in the world, with 245 stations within the 86.9 km2 (34 sq mi) of the city of Paris.