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 islands of Great Britain and Ireland, and most of the railway construction in the Republic of Ireland was undertaken before independence 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 political division of nation states.

• Rail transport in Great Britain discusses rail transport on the larger of the British isles, comprising England, Scotland and Wales. Here, the vast majority of the railway system standardised on the standard gauge of 4 feet 8½ inches. (1435mm)
• Rail transport in Ireland discusses rail transport on the island of Ireland, comprising the Republic of Ireland and Northern Ireland. Here a system using a gauge of 5 ft 3 ins (1600 mm) developed.

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.