The world’s first underground passenger railway used steam trains.

The railway addressed a practical problem: mainline trains brought more people into London, but passengers still needed to cross congested streets to reach the City. Putting a connecting railway below ground offered another route through that bottleneck.

Steam power made an underground passenger service possible before electric traction became established. It also introduced a serious drawback, because smoke, steam and fumes accumulated in the enclosed spaces where trains operated and passengers waited.

The original railway was largely a shallow, cut-and-cover line, not a deep tube bored far below buildings. That distinction explains how Victorian engineers could build it using established construction techniques, despite the disruption caused at street level.

By the middle of the nineteenth century, London's railway growth had created a difficult final stage for many journeys. Arriving at a mainline terminus did not necessarily mean arriving near work or business. Travellers joined the existing road traffic, adding pressure to streets shared by pedestrians, carts and horse-drawn vehicles. An underground connection promised to move people without demanding another continuous strip of surface railway through the city.

Construction began in 1860. Workers opened substantial trenches, built the railway within them and covered much of the route so streets could be restored above. This was the cut-and-cover method. It required extensive interference with the streets during construction, but produced tunnels close to the surface. The finished railway therefore differed fundamentally from the deeper tube railways that later became another defining feature of London's network.

The first section opened to the public on 10 January 1863, linking Paddington with Farringdon Street. Its success demonstrated that passengers would use a railway running below a major city. However, its trains were hauled by steam locomotives. The familiar modern image of electric Underground trains can make that original arrangement seem surprising, but the railway's opening belonged to an earlier stage of transport technology.

Operating steam engines in tunnels created conditions that were far from comfortable by modern expectations. The London Transport Museum describes a persistent fug in the stations and tunnels. Engineers sought ways to reduce the problem, including locomotives fitted with condensers to limit escaping steam. Such measures addressed aspects of steam operation without making the underground environment equivalent to a later electric railway.

Expansion followed, but the complete network did not appear at once. The District Railway began operating in 1868, and the Inner Circle was not completed until 1884. These later developments matter because the phrase 'the Underground opened in 1863' compresses a long process into one anniversary. What opened that January was the pioneering Metropolitan section, rather than every route now associated with the London Underground.

Underground describes the route, not the source of power. The first Metropolitan Railway used steam locomotives beneath the streets, bringing familiar railway technology into an unfamiliar enclosed setting. The resulting smoke problem was a consequence of that combination, not a contradiction in the name.

The first route also addressed a particular urban need: linking main-line terminals with the City. Its success did not mean that the complete modern network, deep tunnels, or electric trains already existed at the opening. Those belong to later stages of development.