Although the Santa Fe was already in the midst of converting freight traffic from steam to diesel during World War II, it procured 25 more steam locomotives in 2-10-4 wheel arrangement in 1944. They were all delivered with oil firing, got the numbers 5011 to 5035 and were also called “War Babies”. Despite a smaller firebox, the performance of the boiler could be kept almost the same. By distributing the weight differently, it was possible to increase the adhesive weight by four tons while the locomotive weight had fallen by four tons. Significantly heavier tenders with eight axles were used again, which held 24,500 gallons of water and 7,000 gallons of oil. This lifted the combined weight of the locomotive and tender to just over the magic one million pound mark.
As late as 1944, measurements of the performance of the first locomotive in the series were made, which led to sensational values. In one test, a five-mile grade of a constant 0.8 percent was climbed by a train of 94 cars weighing about 5,440 tons. From an initial speed of 56 mph, there was still 18 mph left at the end of the incline, where most other locomotives would have come to a standstill long ago. With these performances, the locomotives can certainly be described as the pinnacle of the single-frame steam locomotives, as they were only surpassed by locomotives with split running gear and more coupled axles overall. They, too, made their last commercial journeys in 1957 and stood in reserve for another two years before almost all of them were scrapped. Four examples are still preserved today, none of which are operational.