The locomotives of the class E 91 were heavy freight locomotives for use in the mountains, which had been developed on the basis of the E 77 procured from 1924. In contrast, however, the E 91 featured six powered axles, higher power and lower speed for greater pulling power. Like its predecessor, it was a locomotive with a three-part body, which was mounted on only two long bogies. Above each bogie were two motors, which gave their power to the coupled wheels via a common jackshaft. The structure of the bogies was similar to that of the E 77, but instead of the carrying axles there was a third coupled axle.