* Diesel-electric locomotives: These use large fuel tanks, often measured in gallons or liters, with capacities ranging from several hundred to several thousand gallons (or equivalent liters). The size depends on the locomotive's size, power, and intended operating range between refueling. A smaller switching locomotive might have a much smaller tank than a large mainline freight locomotive.
* Electric locomotives: These don't have fuel tanks at all. They draw power from overhead lines (catenary) or a third rail.
* Steam locomotives: These used to have large water tanks (for steam generation) and a separate tender (a trailing car) to carry coal (their fuel). The capacity varied greatly depending on the locomotive's size and intended use. The coal capacity could be many tons.
Therefore, giving a single "typical" fuel tank capacity for a locomotive is impossible. You'd need to specify the type and model of locomotive to get a meaningful answer.
