In order to explore the extreme limits of the Mallet principle, the Atchison, Topeka & Santa Fe developed the class 3000 in 1911 with the wheel arrangement 2-10-10-2 and a weight of 309 short tons. The goal was to be able to pull 2,000-ton trains with just one locomotive over gradients of 1.2 percent in Arizona. In order to speed up development and production, the running gear for each of the ten examples was taken from two standard Baldwin 2-10-2 locomotives and combined into one locomotive. The existing cylinders were used as rear high-pressure cylinders and Baldwin had larger low-pressure cylinders made for the front bogie. A special development was the flat six-axle tender with a rounded rear section, which was called “Turtleback” and was intended to provide better rear visibility. It contained oil for firing.

A normal boiler of the same length as the class 3000 could not be realized and therefore only the rear half was used as a classic boiler barrel with combustion chamber and smoke tubes. The front half was used as a feedwater heater, superheater and reheater for the exhaust steam from the high-pressure cylinders. All in all, it was found that this boiler did not generate enough steam for the powerful engine and therefore sustained speeds of only 10 to 15 mph were possible.

In view of the low speeds, the locomotives were not used directly to pull heavy freight trains, but had to push other trains uphill. There they were mainly found in California on Cajon Pass and in the famous Tehachapi Loop. Nevertheless, the demolition of the first locomotives back to simple 2-10-2 locomotives began as early as 1915. When the last ones were dismantled in 1918, they had earned the nickname “magnificent failures”. Only the new design of the tender was considered successful and used in later Santa Fe locomotives.