The Fahrpanzer Mobile Fortification Rail Unit
The only surviving specimen of The Fahrpanzer (mobile shielding) is on display at Royal Army Museum of Brussels.
This interesting "canned-operator" concept was created in Germany in the beginning of the last century to ride along rails and packed a 50mm gun in a hand cranked rotating turret.
The Fahrpanzer however was not motorized, and was operated pretty much "Fred Flintstone style" along railroad tracks.