the.com/iron horse

means An old nickname for the steam locomotive or train, coined when people needed a familiar word for an unfamiliar, smoke-breathing machine.

from The phrase rose in the early 1800s, right as steam railways were spreading across Britain and America. People of the time described almost anything by comparing it to what they already knew, and the horse was the standard of power and travel — so a hissing, chugging engine that pulled loads without tiring naturally became an 'iron horse.' Some Native American communities who first saw trains on the plains are also recorded using the same image independently, calling it an 'iron horse' too, likely for the same plain reason: it ran like a horse, but was made of metal and needed no rest.