The Class S 2/5 locomotives operated by the Royal Bavarian State Railways (Königlich Bayerische Staats-Eisenbahnen) included two express train, steam locomotives of American origin which were fitted with Vauclain compound engines. The locomotives had a four-cylinder, Vauclain compound engine, whereby the high-pressure and low-pressure cylinders were located one directly above the other and worked on a common Pleuel connecting rod. The advantage of this design was that no internal gear was needed that would have been difficult to access and which would have needed expensive cranked axles.