London, with a population of about 10.4 million, is one of the biggest commuter cities with an underground train system in Europe. London Underground, better known as the Tube, handles up to 5 million journeys a day, according to operator Transport for London (TfL). Opened in 1863, it is the world's oldest underground rail network, and one of the biggest. A weekend overnight service launched in 2016. TfL runs a joint initiative with police - Project Guardian - aimed at combatting sexual harassment on transport.