The Class S 3/6 steam locomotives of the Royal Bavarian State Railways (later Class 18.4-5 of the Deutsche Reichsbahn) were express train locomotives with a 4-6-2 Pacific (Whyte notation) or 2'C1' (UIC classification) wheel arrangement. Of all the state railway locomotives, these engines are remarkable because they were made over a period of almost 25 years, even during the Deutsche Reichsbahn era. A total of 159 units were manufactured, more than all the other state railway Pacifics taken together. A total of 89 of these locomotives (Series a to i) were built for the Royal Bavarian State Railways and 70 (Series k to o) for the Deutsche Reichsbahn. Common features The S 3/6, designed by the Maffei company under the leadership of engineers Anton Hammel and Heinrich Leppla, was a development of the first German Pacific, the somewhat smaller Baden IV f. Like its forerunner, the S 3/6 had a four-cylinder compound running gear with single-axle drive on the second coupled axle. With the exception of series d and e, that had a coupled wheel diameter of 2,000 mm, the S 3/6 had a coupled wheel diameter of 1,870 mm, that allowed it to be deployed both in hilly terrain as well as on the plains. The permitted top speed of the locomotives was 120 km/h, regardless of the driving wheel diameter. Its performance increased during its production lifetime from Royal Bavarian State Railways Series 0 Maffei delivered the first engine on 16 June 1908, four days later the second, another five prototypes followed until November of the same year, numbers 3601 to 3607. Series a The first official series of ten locomotives were built until September 1909, numbers 3608 to 3617. Series b This series only consisted of the number 3618, construction number 3142, build 1910 especially for the Brussels International Exposition, adorned with brass trim and dark blue bright sheet metal trim. Series c In May and June 1911 another five engines were produced as Series c. All of them above had a driving wheel diameter 1,870 mm and were coupled to 2'2' T 26,2 tenders. Series d and e In 1912, 18 series d and e machines were produced, with a 2,000 mm driving wheel diameter. They were intended specifically for express train service on the relatively flat routes between Munich and Nuremberg as well as between Munich and Würzburg. Unlike the other locomotives, both high- and