Bavarian AA I

The AA I was a prototype used to test ways of saving coal and water by using a lift axle. It was based on the B XI, which was built from 1892 both as a conventional saturated simple locomotive and with a compound engine. The 4-4-0 wheel arrangement of the predecessor was modified by replacing the second coupled axle with a trailing axle. For this purpose, the lift axle with small wheels was used between the lead bogie and the first coupled axle, which was driven by a second engine with two smaller cylinders.

With this configuration, it was possible under most conditions to drive with only one driving axle, thus saving coal and water. The lift axle was raised using a pressure cylinder because running along with the additional engine being turned would have required more power. If more power or traction was needed, the lift axle was lowered and some of the steam was fed to its cylinders. Although the locomotive with this system was economical and reliable on a day-to-day basis, the complicated engine required more maintenance than with conventional steam locomotives. So it was decided to continue to procure the B XI.

After an accident in 1907, the AA I was converted into a 4-4-0 locomotive, making it look like its predecessor. In contrast to this, it received a superheater. With this powerplant, it was able to last the longest of all Bavarian 4-4-0 engines in service with the Reichsbahn and was therefore not retired until 1933 as road number 16 861.