Steam locomotives often haul a tender, which is a special railroad car designed to hold the locomotive's fuel and water. Steam locomotives consume large quantities of both. Locomotives that do not have tenders and carry all their fuel and water on board the locomotive itself are called tank locomotives.
One reason not to carry the fuel and water aboard the locomotive is that the rate of consumption of both is such that it is hard to carry enough for an extended range. Another is the desire to keep the locomotive's weight near constant, so that its hauling abilities remain consistent (since they are dependent on weight on driven wheels multiplied by a coefficient of friction).
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