For the Seetalbahn, which was nationalized in 1922, the SBB ordered three freight locomotives that were to be delivered in 1926. Since the route was electrified with the unusual system of 5,500 volts and 25 hertz, the locomotives had to be able to be used with this system. Due to the planned conversion to the system with 15,000 volts and 16⅔ Hertz that is common in German-speaking countries, however, they were designed in such a way that they could run under both systems.

The design of a crocodile was used, i.e. the locomotives stood on two bogies, each of which had a motor in a small hood. There was a body between the bogies, which essentially contained the driver's cabs and the transformer. The locomotives, known for this reason as the “Seetal crocodile”, were built using as many assemblies as possible from the electric shunting locomotive Ee 3/3 due to the small number of units. The two bogies, each with a traction motor and Winterthur diagonal rod drive, corresponded as far as possible to the Ee 3/3.

Although the Seetalbahn had meanwhile been nationalized, the three De 6/6s were used almost exclusively on this and the directly adjoining lines. The maximum speed of only 50 km/h also prevented use on main lines. After the conversion of the power system on the Seetalbahn, the equipment that was only necessary for the old power system was removed. The locomotives were retired in 1983 and two out of three were scrapped. The third was sold to the Oensingen-Balsthal-Bahn and was initially used there. After a short circuit, an association was founded to repair the damaged equipment. This locomotive has been operational again since May 2015.