Prior to the advent of the steam railway, London was a metropolis of just over 1 million people. It was well served by both canals and turnpikes connecting to other parts of Great Britain. Internally, there were omnibus services. The London & Greenwich Railway was the first of many railways to reach London, with the first section opening in 1836 and being completed in 1838, making it possible to reach Greenwich in twelve minutes instead of the hour required by horse-drawn omnibus or steamboat. Famously built on a viaduct, the route was initially paralleled by a tree-lined boulevard that operated as a toll road, serving those unwilling to pay rail fares. However, the toll road was disbanded when the viaduct was widened to enable more frequent services to the densely populated urban core, ultimately growing from two tracks to eleven.
Soon many other railways sought to connect to London. To avoid disruption in the core, a Royal Commission on Railway Termini, appointed in 1846, drew a box around central London and decreed no line shall enter the cordon. [This box resembles the congestion charging zone adopted in the early 21st century, which aimed to reduce cars, rather than prohibit trains]. The result was railway terminals locating on the edges of the central region. London, like many cities, has no unified railway station, as the North, South, East, and West lines have no common intersection. The problem is worse though in London, as even lines from the north run by different organizations would be build adjacent (St. Pancras/ Kings Cross), or nearly adjacent (Euston), stations without convenient interchange. Later (between 1858-60) some penetrations of the box were permitted by Parliament, but most of the City of London (the original walled city where the financial district still lies) remained untouched. While preventing railways from severing the most densely populated part of the city, which would have been expensive for both the railways and the city, it created a need for a connection between the termini to allow transfers. The Metropolitan Railway, a private concern like all railways of the era but with some support from the Corporation of the City of London, was approved by Parliament in 1854. It aimed to connect the northern termini (Paddington, Euston, St. Pancras, King’s Cross, and Farringdon, which was later added to the plan) to ease movement for through travelers.
The trends in the City of London were quite different from the rest of London. The City of London has seen a long trend of depopulation from 1851 (prior to the first Underground line) and for many years saw increasing employment, lending support to the notion that the railways, especially the Underground, enabled decentralization of residences and concentration of employment.
The Metropolitan Railway opened in January 1863, and was extremely successful. Clearly the market was much larger than inter-line transfers. The firm paid dividends throughout its life. Accounting in the early years of the Metropolitan Railway, especially prior to the Regulation of Railways Act of 1868, was a bit dodgy, and dividends were reportedly paid out of capital. To quote Jackson (1986) p. 38, describing the era of 1865, “It was . . . a house of cards, a precarious game in which the level of dividend was kept up at all costs, by finding money from somewhere, with no regard to sound accounting or financial rectitude.”. Emulation is the proof of success. Many new railway lines were proposed, the 219 London-area railway bills brought before Parliament during the period 1860-1869 totaled 1420 km (882 miles).
Some of those lines were proposed prior to the opening of the Metropolitan, indicating the smell of success was in the air, though the peak years were between 1863 and 1866, following closely on the heels of the Metropolitan’s opening. The most important of these was the Metropolitan District Railway (later called the District line), which ran just north of the River Thames, but south of the Metropolitan, connecting a number of the southern railway termini (Victoria, Charing Cross, Blackfriars, Cannon Street). Proposals for what became the Circle Line service linking the Metropolitan and District (roughly inscribing the box described above) were quickly proposed, but the two lines were not connected on both ends until 1884. Both the Metropolitan and District lines were constructed using cut and cover techniques. Later lines, from the City and South London Railway (first section opened in 1890) onwards, generally used deep-level tunneling techniques to avoid disruption of city streets, existing railway lines, and public utilities when they needed to be below grade. Outside the Circle Line however, the railways could emerge above ground and competed fiercely in some markets, while operating unfettered in others, to provide suburban services. In some cases this involved building new lines, in others it involved acquiring running rights on (or ownership of) existing lines. The development of suburbs was a way to develop traffic for lines that in the city, though profitable, were operating below maximum capacity, and thus maximum profitability.
Adapted from

• Levinson, David (2008) The Orderliness Hypothesis: Does Population Density Explain the Sequence of Rail Station Opening in London? Journal of Transport History 29(1) March 2008 pp.98-114.

Also see:

• Levinson, David (2008) Density and Dispersion: The Co-Development of Land use and Rail in London. Journal of Economic Geography 8(1) 55-57.