Experiment

Experiment was a steam locomotive built by Timothy Hackworth in 1826 for the Stockton and Darlington Railway, one of the small fleet of engines that worked the world's first public steam-operated railway in its pioneering early years. The Stockton and Darlington Railway had opened on 27 September 1825 with Stephenson's Locomotion No. 1 hauling the inaugural train, and Experiment followed the following year as the S&DR sought to expand and improve its locomotive fleet for the growing traffic between the Durham coalfields and the River Tees at Stockton.

Timothy Hackworth had been appointed as the Stockton and Darlington Railway's resident engineer in 1825, responsible for maintaining and developing the locomotive fleet in the difficult early years when the engines were constantly breaking down and the railway's management was repeatedly questioning whether steam traction was really preferable to horse power. Hackworth was a skilled practical engineer — trained under William Hedley at Wylam — and his efforts to improve the reliability and performance of the S&DR's engines were essential to the railway's survival as a steam-operated line.

Experiment incorporated several improvements over Locomotion No. 1, including a return-flue boiler arrangement that gave better heat transfer and more efficient steam generation, addressing one of the chronic weaknesses of the original Stephenson-pattern locomotives. The return-flue boiler — in which the fire-tube passed forward through the boiler, doubled back, and exited at the same end as the firebox — gave more heating surface for a given boiler length, a significant advantage when early boilers were persistently struggling to generate sufficient steam for sustained working.

Experiment itself was eventually withdrawn and scrapped, but Hackworth's subsequent development of the return-flue principle produced the Royal George of 1827, which was a considerably more successful locomotive and demonstrated that the S&DR could be reliably operated by steam. No physical record of Experiment survives beyond the S&DR's engineering records and Hackworth's own notes.