Early steam engines were developed well before the locomotive steam engine, which is generally what we associate with the term "steam engine". I don't think you

Early steam engines were developed well before the locomotive steam engine, which is generally what we associate with the term "steam engine". I don't think you can overstate the importance of the technological development in the 19th century on these developments. The US presented a new need that I don't think really existed before: crossing vast distances over land, eventually all the way from the Atlantic coast to the Pacific coast. IIRC the population of the US was 2 million in 1800 and 50 million in 1900. Others have mentioned precision engineering. This was an important factor and had its origin in manufacturing cannon bores. But a bigger factor (IMHO) was steel. Steel existed before the mid-19th century but it was incredibly expensive (more expensive than gold) and relatively low volume. This all changed with the Bessemer process that allowed the mass production of inexpensive steel. It made Andrew Carnegie in particular incredibly wealthy in the process. Trains existed before cheap steel but builds of railroads and trains absolutely exploded in the wake of cheap steel. This also led to the shift from wooden sailing ships to (ultimately) steel-hulled boats with engines. So prior to trains, what were the potential use cases for a steam engine? Automobiles seem unlikely (given weight and size and the ubiquity and utility of horses). Ships? Maybe. But sailing is effective and fires tend to be bad news for ships. So that really leaves factories, mills, etc. Mills in particular often used hydro power (ie a river turning a wheel). You could build them in locations with such features. Labor was also relatively cheap (and largely free in some cases ie slaves). Ultimately I don't think it's one thing and it's hard to separate these factors cleanly. It becomes a chicken and egg problem. Advancement can often fuel each other. It does seem like long distance transportation ultimately led to at least popularizing and mass producing steam engines.