The separate vehicle coupled behind a steam locomotive to carry its coal and water is a tender.
A tender extended a locomotive’s operating range by carrying the supplies that the engine consumed rapidly. Its fuel bunker held coal, wood, or oil, while a much larger tank stored water for making steam. The fireman replenished the fire and fed water into the boiler during the run.
Tenders were especially important because steam locomotives used enormous quantities of water. A locomotive carrying its own supplies in tanks mounted on the engine is instead called a tank locomotive. That distinction is a common source of confusion: a tender is not simply any vehicle hauled behind a train, but a specialized supply vehicle attached to the locomotive.
Other tender designs served particular purposes. Some carried extra water, some condensed exhaust steam, and corridor tenders included a passageway for crew movement. In ordinary railway terminology, however, “tender” means the locomotive’s fuel-and-water vehicle.