The Coalbrookdale company then built a rail locomotive for Trevithick, but little is known about it, including whether or not it actually ran. To date, the only known information about it comes from a drawing preserved at the Science Museum, London, together with a letter written by Trevithick to his friend, Davies Giddy. The design incorporated a single horizontal cylinder enclosed in a return-flue boiler. A flywheel drove the wheels on one side through spur gears, and the axles were mounted directly on the boiler, with no frame. On the drawing, the piston-rod, guide-bars and cross-head are located directly above the firebox door, thus making the engine extremely dangerous to fire while moving. This is the drawing used as the basis of all images and replicas of the later

"Pen-y-darren" locomotive, as no plans for that locomotive have survived.

Source: Wikipedia