Merthyr Tydfil (mûrˈthər tĭdˈvĭl) [key], town (1981 pop. 38,893) and county borough, 43 sq mi (111 sq km), S Wales. Located on the Taff River, the town is connected to Cardiff by canal. It has ironworks and steelworks. After World War II, light industries were stressed to revive the economy. Textiles, clothing, and leather goods are also made. The name Merthyr Tydfil comes from the tale of the martyrdom of St. Tydfil, a Welsh princess killed in the 5th cent. Richard Trevithick built the world's first steam railway locomotive there in 1804.