The Deutsche Reichsbahn-Gesellschaft's BR 01 steam locomotives were the first standardised (Einheitsdampflokomotive) steam express passenger locomotives built by the unified German railway system. They were of 4-6-2 "Pacific" wheel arrangement in the Whyte notation, or 2′C1′ h2 in the UIC classification. The idea of standardisation was that it would reduce maintenance costs; i.e. if a BR 01 whose engine shop was in, say, Berlin broke down in Dresden, instead of having to ship the necessary part from Berlin and take the locomotive out of service, a part from the Dresden shop could be used as all of the engines, parts, and workings were exactly the same and produced nationwide. Thus it was a "standard" product for engine shops. History The firms of AEG and Borsig, who were the main manufacturers of these engines, together with Henschel, Hohenzollern, Krupp and BMAG previously Schwartzkopff, delivered a total of 231 examples of this Einheitsdampflokomotive between 1926 and 1938 for the fast passenger services of the Deutsche Reichsbahn. To begin with, 10 locomotives of this class were built with two-cylinder engines for comparison purposes alongside a similar batch of 10 engines of their sister Class 02, which had four-cylinder compounding. Extensive measurement and trial runs were conducted, but after lengthy discussions the controversial decision finally fell in favour of the two-cylinder configuration, which was simpler to maintain but less powerful and less economical than the four-cylinder compounds. The first Class 01 locomotive that went into service was not 01 001, but 01 008, which is preserved today in the Bochum-Dahlhausen Railway Museum. The mass production of Class 01s was somewhat delayed at first because in the 1920s there were neither enough routes with the necessary axle load of 20 tons nor sufficiently large turntables. Not until the beginning of the 1930s did the Class 01 become the predominant express train locomotive of the Deutsche Reichsbahn. By 1938 there were 231 Class 01 locomotives available for the prestigious express train duties. Another 10 four-cylinder Class 02 locomotives (01 111, 01 233–241) were converted to two-cylinder Class 01 models between 1937 and 1942. To accommodate for the many routes with axle load restrictions too low for Class 01 service, in the early 1930s, a third variant was created: the Class 03 designed with a two-cylinder engine and axle load of 18 ton, of which no less than 298 were built. Launched in 1939, the three-cylinder DRG