The railway system of Great Britain, the principal territory of the United Kingdom, is the oldest in the world. The system was originally built as a patchwork of local rail links operated by small private railway companies. These isolated links developed during the railway boom of the 1840s into a national network, although still run by dozens of competing companies. Over the course of the 19th and early 20th centuries, these amalgamated or were bought by competitors until only a handful of larger companies remained (see railway mania). The entire network was brought under government control during the First World War and a number of advantages of amalgamation and planning were revealed. However, the government resisted calls for the nationalisation of the network. In 1923, almost all the