In order to be able to burn anthracite coal efficiently, class AAn was developed from class K (later D6). The K already had a wide firebox above the drivers, which was optimized here for the use of anthracite. There were 58 locomotives with a coupling wheel diameter of 68 inches, which became class D7 from 1895, and 61 with a coupling wheel diameter of 62 inches, which became class D7a.