The high-pressure steam engine was invented about the same time by Richard Trevithick in the UK and Oliver Evans in the US. Neither man knew about the other.
Trevithick, a mining engineer, built a high-pressure steam-powered car, the “Puffing Devil,” in 1801, taking it for a ride around town, picking up friends. He left it at a pub where they went to celebrate his innovation. While drinking they failed to notice the fire from the steam boiler and the car burnt down.
James Watt, the inventor of the condensing (low pressure) steam engine, believed Trevithick’s high-pressure steam engines dangerous. After one of Trevithick’s early high-pressure steam engines, used as a pump, exploded and killed two people Watt urged Trevithick’s prosecution for murder. There are rumors that Trevithick’s and Watt’s lead engineer Richard Murdoch were neighbors and may have secretly collaborated building the engine.
Starting in 1802, Trevithick created several high-pressure locomotive steam engines that ran on tracks. These were the earliest trains but none realized commercial success.
Trevithick eventually moved to South America to work on trains for mines where he met railroad baron George Stephenson, who gave him money to return to the UK.
Historians disagree whether Trevithick died in poverty or merely with little money. He did not meaningfully profit from his engines.
American Oliver Evans (automated mills, refrigeration) simultaneously worked on high-pressure steam engines in the US. He built an amphibious vehicle with his high-pressure engine, that “walked” from his shop to the docks, and into the water. It worked as a dredger. Like Trevithick, his high-pressure engines initially failed to gain commercial interest. Later in life, after working through issues with his mill patents, Evans built a steam engine business with his sons. They produced about 100 high-pressure steam engines.