
The Metropolitan Railway opened in London in January 1863, running steam trains in a shallow cut-and-cover tunnel between Paddington and Farringdon to carry commuters into the congested City. It was the world's first underground urban railway and moved roughly 9.5 million passengers in its first year. Later lines used deep tube boring and, from the 1890s, electric traction that removed the smoke problem.
Grade separation let a railway carry far more people through a dense city than any street-level mode, decoupling capacity from surface congestion. It enabled the high-density metropolis and concentrated growth and land value around stations, a pattern still central to urban form.