In 1804, Merthyr witnessed the first ever steam locomotive journey, as Trevithick's 'Penydarren Locomotive' travelled down the valley, through this tunnel, and on to Abercynon.

Samuel Homfray of the Penydarren Ironworks invited Cornish engineer, Richard Trevithick to his works in 1803 to install high-pressure stationary engines. After these engines were installed Homfray became interested in another of Trevithick's designs, namely that for a steam railway locomotive that he had recently drawn up for the Coalbrookdale Works. Homfray thought it would revolutionise the transportation of goods.

Work on the construction of the locomotive started in October 1803. Trevithick wrote:
"Last Saturday night we lighted the fire in the tram Waggon and worked it without the wheels to try the engine, and Monday we put it on the tram road. It work'd very well, and ran up and down with great ease, and very manageable."

The Wager

Richard Crawshay of the Cyfarthfa Ironworks expressed doubt as to the ability of such a locomotive being able to work. So strong was his conviction that he made a bet with Homfray of 500 guineas that the locomotive and its load could not reach Abercynon and return with its empties.

The Journey

On the 21st February 1804:
"we proceeded on our Journey with the engine; we carry'd ten tons of iron, five wagons, and 70 men riding on them the whole of the journey...we had to cut down some trees and remove some rocks out of the road. The engine, while working, went nearly 5 miles pr hour...On our return home about 4 miles from the shipping place of the iron, one of the small bolts that fastened the axel to the boiler broak, and let all the water out of the boiler which prevented the engine returning until this evening."
The delay in returning to Penydarren made no difference, the locomotive was a success.

History has not been kind to Trevithick, although he undoubtedly succeeded in producing the world's first steam locomotive he continues to be overlooked.