Which country built the world's first underground railway?

The United Kingdom built the world's first underground railway: London's Metropolitan Railway opened in 1863.

The line began passenger service on 10 January 1863, running between Paddington and Farringdon. It was created to relieve severe congestion on London's roads and connect major main-line stations with the City. The railway was built largely by the cut-and-cover method, in which trenches were excavated, roofed over, and restored at street level.

The first trains were hauled by steam locomotives, so early passengers encountered smoke, heat, and poor ventilation. Despite that discomfort, the railway was an immediate success and carried tens of thousands of passengers on its opening day. Its success encouraged the construction of additional underground lines, including the District Railway.

A common mix-up is confusing the first underground railway with the first deep-level electric “Tube” railway. London’s original system was shallow and steam-operated; deep-bore electric lines came later. The Metropolitan Railway eventually became part of the modern London Underground network.