The world's first underground railway was built in London, the capital city of England. It opened in 1863 and is now known as the London Underground or the Tube. The initial line was called the Metropolitan Railway and ran between Paddington and Farringdon. It was a response to the city's growing traffic congestion and population density. Today, the London Underground is one of the busiest metro systems in the world, with 11 lines covering 402 kilometers (250 miles) and serving over 5 million passengers daily.