Donkey's were used to power things like grist mills, pulling a wheel, around and around with gears to transfer the energy to a stone roller. When stationary boilers began to be used for this kind of work, they were called steam "donkeys"...

Horses were more commonly used for pulling wagons and coaches, and when steam engines started pulling wagons and coaches (on tracks), they were referred to as iron "horses". And, depending on the sources you want to believe, although without any specific citations of early use in say publications (?), according to most of these sources, it was believed to be into wide common usage, both in the US and Britain by the 1840's.

It had nothing to do with the transcon nor any need to explain the choo-choo, using "pigeon English", to the Indians, or if you are really troubled by that term, the true "Native" Americans.

And, it takes about two minutes on google to find that out....?