One of the books I'm writing is set about 1805-1810. The Heroine is besotted with the idea of using steam power. I thought I'd put down a few notes on the background. Just to set the record wrong on a few critical points.
Watt's patents were still in force at the time. His big improvement was the use of an external condenser. The previous state of the art machines (Newcomen steam engines) used a jet of cold water as part of the engine cycle to cool the steam in the piston. Watt's external condenser doubled the efficiency to all of 4% or so.
Watt engines worked backwards from the way we'd think. The steam pressure was barely above atmospheric pressure, and it was the vacuum produced by the condensing steam that made the piston move. This meant the engines were big and "static"
The amount of work you could get from the engine was proportionate to the size of the piston, since the pressure doing the work was nearly constant. Low pressure also meant that the piston seals and boiler construction weren't exactly critical components.
Trevithick and other innovators were forced to go to "Live steam" or high pressure because of Watt's patent on the external condenser. High pressure in those days was several atmospheres. This mean you could have smaller, mobile engines. Trevithick took advantage of that to make both rail engines and things that vaguely resembled cars. One of his inventions, after a boiler explosion, was a safety valve. Smaller pistons and higher pressures meant the engines worked the way we'd imagine, with the steam pressure pushing the piston. (Once Watt's patents expired they added an external condenser.)
The materials available for construction were basically wrought iron and brass or copper. Cast iron is unreliable under tension due to having microscopic cracks formed during cooling. The iron could be hammered to approximately the correct shape and size, but would have to be machined for a precision fit. Even then it would take careful hand fitting to make things work. (Hence the phrase a "steam fitter".) Modern precision, modern materials, and controlled manufacturing are more than a century away. For example at the start of WW1 about 1/2 the steel receivers of 1906 Springfield rifles were rejected as too brittle due to "eyeballing" the quality of the steel and temperatures during manufacturing. This meant that "high tech" could sometimes, and did quite often, involve high risk.