The term "hostler" originated with the people who took care of four-legged horses, and it got carried over to those who took care of iron horses. At the Strasburg Rail Road, the hostler gets the locomotive hot in the morning, greasing, oiling, tending the fire, and filling the tender with coal and water -- and adding the proper chemicals to the water to treat pH and dissolved solids, as Ross will shortly do as he climbs up on the tender.