Merthyr Tramroad / Penydarren Ironworks

The Merthyr Tramroad (also called the Penydarren Tramroad) is the most historically significant industrial railway in the world — the 9¾-mile horse-worked iron tramroad in South Wales where Richard Trevithick demonstrated the world's first steam locomotive journey on 21 February 1804. The tramroad had been built around 1802 to connect the Penydarren Ironworks at Merthyr Tydfil with the Glamorganshire Canal at Abercynon, serving as a workaday industrial freight line for the iron and coal output of one of the most productive ironworking districts in Britain.

On 21 February 1804, Trevithick's Penydarren locomotive — a high-pressure steam engine mounted on a wheeled frame — hauled 10 tons of iron, 70 men, and five wagons over the Merthyr Tramroad, completing the world's first recorded journey of a steam locomotive on iron rails. The ironmaster Samuel Homfray had wagered 500 guineas with a rival that Trevithick's engine could haul iron along the tramroad, and won his bet — but the locomotive's weight broke several of the cast-iron tramroad plateway rails, and after a few further trips Trevithick's engine was put to work as a stationary blowing engine at the ironworks rather than continuing as a locomotive.

The Penydarren demonstration was a proof of concept rather than a commercial success, and Trevithick's pioneering work was not immediately followed up by the ironmasters of South Wales. It was left to the north-east England colliery engineers — William Hedley at Wylam and George Stephenson at Killingworth — to develop the practical steam railway locomotive over the following decade. But Trevithick's 1804 journey at Merthyr established that steam locomotion on iron rails was physically possible, a demonstration without which everything that followed would have been delayed by years or decades.