They call it a whale-back
Here's a rear-quarter view of Southern Pacific narrow gauge locomotive #18, running in the Silverton, Colorado yard as she prepares for a Trains Magazine photo shoot. This image provides a clear view of the unusual "whale-back" tender design that was used on some oil-burners. So named because the rounded sides and top resemble the back of a whale, these tenders were optimized for carrying liquids, including fuel oil and water. Back in the day, an engine like this would typically be burning heavy bunker oil, and the tender design included steam lines from the boiler, for the purpose of heating the oil to make it flow and atomize more easily. Today, most oil-burners, including the 18, burn waste motor oil, which flows and atomizes pretty easily, even in relatively cold temperatures.
They call it a whale-back
Here's a rear-quarter view of Southern Pacific narrow gauge locomotive #18, running in the Silverton, Colorado yard as she prepares for a Trains Magazine photo shoot. This image provides a clear view of the unusual "whale-back" tender design that was used on some oil-burners. So named because the rounded sides and top resemble the back of a whale, these tenders were optimized for carrying liquids, including fuel oil and water. Back in the day, an engine like this would typically be burning heavy bunker oil, and the tender design included steam lines from the boiler, for the purpose of heating the oil to make it flow and atomize more easily. Today, most oil-burners, including the 18, burn waste motor oil, which flows and atomizes pretty easily, even in relatively cold temperatures.