As an alternative to the eight class Z Kitson-Meyer rack locomotives, Maschinenfabrik Esslingen supplied these two locomotives. They had a front bogie with three carrying axles and three rack pinions for the Abt system. The rear bogie had four coupled axles for the adhesion drive. Initially, a condensation device was installed, which was soon removed.
They were numbered 9 and 11 from the factory, the 9 was soon renumbered to 10. The number 10 derailed after a short time in the mountains between Juncal and Portillo, where salvage was uneconomical. It was left in place and no further locomotives of this type were built.