Trevithick built a full-size steam road carriage in 1801 on a site near the present day Fore Street at Camborne, which was also known as Camborne Hill. He named the carriage 'Puffing Devil' and, on Christmas Eve that year, he demonstrated it by successfully carrying several men up Camborne Hill and then continuing on to the nearby village of Beacon with his cousin and associate, Andrew Vivian, steering. This event is believed by many to be the first demonstration of transportation by (steam) auto-motive power and it later inspired the popular Cornish folk song "Camborne Hill". However, others suggest that Nicolas-Joseph Cugnot may have an earlier claim with his steam wagon of 1770, or even that a steam powered car built in 1672 by Ferdinand Verbiest was the first steam powered car. During further tests, Trevithick's carriage broke down 3 days later after passing over a gully in the road. The carriage was left under some shelter with the fire still burning whilst the operators retired to a nearby public house for a meal of roast goose and drinks. Meanwhile the water boiled off, the engine overheated and the whole carriage burnt out, completely destroying it. Trevithick however did not consider this episode a serious setback but more a case of operator error.
In 1802 Trevithick took out a patent for his high pressure steam engine.
Anxious to prove his ideas, he built a stationary engine at the Coalbrookdale Company's works in Shropshire in 1802, forcing water to a measured height to measure the work done. The engine ran at forty piston strokes a minute, with an unprecedented boiler pressure of 145 psi. The company then built a rail locomotive for him, 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, and a letter written by Trevithick to his friend, Davies Giddy. This is the drawing used as the basis of all images and replicas of the later Penydarren locomotive, as no plans for that locomotive have survived.