Underground World: Tackling Barcelona's metro thieves
4 April 2013 Last updated at 07:01 BST
Whether you call it the Metro, the Tube or the Subway - underground transport makes the world go round.
BBC News has been touring the globe's train networks to find out what it's like to be a commuter in some of the world's biggest cities.
Laura Trevelyan looks at New York's "dirty" subway and Pascale Harter examines Barcelona's underground thieves.