I'd say instead of a peter cooper locomotive, maybe some kind of a Trevithick locomotive would be better. It was the first steam powered locomotive and it's relatively small. (and in my view it also fits VS's style better and could be made even smaller, enough to fit in a 3x1 tunnel).
The problem is that Tyron said he only wanted to push steam until the Newcomen engine (invented in 1712). This locomotive uses a later steam engine design and was invented in 1804. We'll have to see what the VS team does in terms of powering minecarts; I certainly would prefer locomotives over powered rails, but the methods on which these would be powered remain to be seen. If not steam, I could see a Jonas contraption being the driving force of a locomotive, being powered with temporal gears (or a new item/concept altogether).
