Number 41 “Dart” of the Stockton & Darlington Railway was an 0-4-0 locomotive built by Timothy Hackworth at Shildon. It had inside frames with outside auxiliary frames, inside cylinders and valve gear and outside coupling rods. When built, it had wheels of three and a half feet, cylinders of 12 by 18 inches and a boiler pressure of 60 psi. When displayed at the S&D's 50th anniversary in 1875, it had been rebuilt to cylinders of 13 by 16 inches and 100 psi.