 They were the first class of locomotives to use superheating, a process in which the steam leaving the boiler is re-heated, resulting in better efficiency.The railway procured a total of 1,027 locomotives of this class up to 1904, and they were stabled at almost all locomotive depots ("Betriebswerk" or "Bw"), making them the most numerous German express train locomotives.In addition, several hundred units were manufactured with smaller driving wheels - dubbed the P4 class.