The United Kingdom consists of Great Britain and Northern Ireland, and previously consisted of Great Britain and the whole of Ireland. Rail transport systems developed independently on the two island masses of Great Britain and Ireland, and most of the railway construction in the Republic of Ireland was undertaken before the creation of the Irish Free State in 1922. Thus, the logical division to discuss the history and present-day state of railways in these areas is by geographical division, rather than the nationalist division of nation states.
Additionally, there are rail systems in two Crown Dependencies, namely:
None of the British Overseas Territories have railways at present, although some, such as Bermuda were historically served by railways. Gibraltar is accessible by the Spanish and Moroccan rail systems. Hong Kong was the last dependent territory to have an extant rail system.
Similarly, for the history of rail transport, rather than the current situation (described in the above articles), see history of rail transport in Great Britain and history of rail transport in Ireland. The United Kingdom, despite its island geography, runs three separate cross border train services: