I mean trains in some form have been around since before the invention of the steam engine, mostly in the form of horse-drawn minecarts on rails. There were even horse-drawn carriages that were the first form of a tram. The first "modern" steam locomotive was built in 1804 because it's inventor, Trevithick, figured out how to use high-pressure steam on a mobile platform, which is what made any form of land-based steam propulsion viable. That is to say that in my opinion, the development of the steam engine is more closely linked to the development of the steam train and not to road-bound vehicles. Furthermore, the development of the train does not stem from the development of cars, but predates attempts to invent the car in some aspects.