The article you linked doesn't support your anecdote about the root of the Shire Horse. It describes their history dating back centuries before railways. Their biggest use seems to have been hauling material to and from ports, not trains.
(I've owned a Percheron, and have known some Shires.)
(relatives have owned Friesian's and Clydesdale's and Norwegian Fjording Horses, but it's neither here nor there)
To advance; to further; to prefect; to make to increase; to promote the growth of. “We must develop our own resources to the utmost. Jowett (Thucyd).”
Rather than develop as in the contemporary software engineering sense synonymous with create.
In this sense, the breed was further developed to serve railway terminals from the original breed created to service maritime ports.
So I think “developed” is fairly appropriate here, though “adapted” might have been more clear.
In engineering we are accustomed to getting involved early in the creation process, and our usage of "develop" reflects this bias.
Outside of that bubble, "develop" is very explicit that the thing already exists. For example, developing a musical theme, a muscle, or a country.