A steam wagon, sometimes known as a steam lorry in Great Britain, was a steam-powered truck designed for road use. They were prominent for many years before the advent and streamlining of gas-powered engines that could produce more power and haul more weight. Taxes and other regulations also reduced the use of the steam wagon, and by the early to mid-twentieth century, were all but obsolete. Two types of steam wagons existed: undertypes and overtypes. They were distinguished by the position of the engine in relation to the boiler unit inside the truck.
The design of the steam wagon was very similar to a traction engine, which also ran on steam power. The difference between the steam wagon and the traction engine was the difference by which goods were hauled: the traction engine hauled a cart behind it, while the steam wagon featured an attached truck bed or cart that was not hauled behind but was rather part of the truck itself. The earliest versions of the wagon featured wooden wheels like those of a carriage, though as they become more commonly used, solid rubber tires were used. Later on, pneumatic tires like modern-day car tires were used as well.
While the steam wagon did not use gasoline, thereby avoiding that type of pollution, the steam powered engine did produce harmful smoke that caused pollution and reduced air quality. This was ultimately one of the reasons for the demise of the design, as gasoline-powered engines were more efficient and powerful while producing the same amount or less pollution. This was not as big a concern as it became by the end of the 20th century and beginning of the 21st century, though it was a contributing factor to lower air quality.
At first, steam wagon models were faster and more powerful than gas-powered versions, which is why the wagon stayed around as long as it did. Taxes placed on the steam models, however, began to make them uneconomical choices, and while taxes on gasoline were initially quite high, they became lower, making gas-powered engines more common. As time passed, freight companies began to abandon the expensive steam wagons in favor of gas-powered models that they could purchase at a lower price and maintain for less money as well. Improvements in the design of the gas-powered models also made them more competitive with the performance of steam models.