The line was opened in 1865 to carry slate from the quarries at Bryn Eglwys to Tywyn and was the first narrow gauge railway in Britain authorized by the Act of Parliament to carry passengers using steam haulage. Despite severe under-investment, the line remained open, and in 1951 it became the first railway in the world to be preserved as a heritage railway.

A heritage railway or heritage railroad (US usage) is a railway operated as living history to re-create or preserve railway scenes of the past. Heritage railways are often old railway lines preserved in a state depicting a period (or periods) in the history of rail transport.

Since preservation, the railway has operated as a tourist attraction, expanding its rolling stock through acquisition and an engineering program to build new locomotives and carriages. In 1976, an extension was opened along the former mineral line from Abergynolwyn to the new station at Nant Gwernol. In 2005 a major rebuilding and extension of Tywyn Wharf station took place, including a much-expanded facility for the Narrow Gauge Railway Museum, and in 2021 the railway was designated a World Heritage Site as part of the slate landscape of north-west Wales.

The fictional Skarloey Railway, which formed part of The Railway Series of children’s books by The Rev. W. Awdry, was based on the Talyllyn Railway. The preservation of the line inspired the Ealing Comedy film The Titfield Thunderbolt.

The Talyllyn Railway remains a successful and popular tourist attraction. The original 1860s locomotives and passenger stock still run regularly alongside the roster of more modern rolling stock. The railway is promoted as one of The Great Little Trains of Wales, a joint marketing scheme launched in 1970 that encompasses ten narrow gauge railways in the country, mostly found in north and mid-Wales.

According to Wikipedia