I've never seen anything like this before. It was an experimental Bavarian locomotive built 1895. Apparently the "booster axle" was used to assist in starting the train, and perhaps disengaged the rail after the train got moving.
http://www.worldrailfans.info/...amExperimental.shtml (first loco in the list)
It was rebuilt into a 4-4-0 in 1908.