The SBB needed powerful mountain freight locomotives especially for the electrification of the Gotthard railway. The specifications specified a whole range of requirements. Among them was that the entire Gotthard route from Goldau to Chiasso and back was to be completed in 28 hours with a train weighing 860 tonnes. A pusher locomotive was allowed to assist on the steepest parts. In addition, 300 tonnes should be able to be pulled at 50 km/h at 2.6 percent and an additional output of 20 percent should be able to be coped with over a period of 15 minutes. In addition, a recuperative brake was part of the mandatory equipment.
With the Fc 2x3/4, the later Ce 6/8I, a prototype for this task had already been created. However, problems with the bogies were feared due to the high tractive forces on the mountain, so that a different design was finally developed. The solution was two individual running gears that were closely coupled to each other and carried a body that did not transfer any tractive forces. This design earned them the well-known nickname “Crocodile”.
The chassis groups each consisted of three coupled axles and one leading axle. The leading axles were designed as Bissel axles and had a lateral play of 83 mm, while the middle coupled axles had a play of 25 mm. The hoods, each with a double motor and other equipment, were mounted on the chassis. The power of the motors was transmitted to the outer coupled axle via a reduction gear and a triangular connecting rod. The locomotive body with the driver's cabs and the transformer lay between the two chassis and had a pivot that could be moved lengthways so that it did not transmit any lateral forces.
Schematic drawing with dimensions
Locomotive Magazine, December 1920
For the electric part, the 33 locomotives showed some differences among themselves. The transformer tray on the first locomotives was air-cooled via cooling fins, while the later ones got their own oil cooler. The tap changer also had a different design depending on the year of manufacture. Since the Gotthard railway was initially only operated at half voltage due to the soot from the steam locomotives that were still running, a switch to 7,500 volts was made possible. As required, a dynamic brake was installed that could feed power back into the overhead line. Electric train heating was also fitted, although use in front of passenger trains was rare.
Since the Gotthard railway was not fully electrified when they were delivered, the locomotives were used right from the start on the Bern-Thun-Spiez route for a short time. They were later used almost all over Switzerland, but primarily on the Gotthard. From 1941, 13 machines were converted for a speed of 75 km/h and then consequently renamed the Be 6/8II.
When the Ae 6/6 was introduced in the 1950's, the Crocodiles were moved to flatter regions. Later they were used as shunting locomotives or in front of slow bulk goods trains. The first ones were retired in 1968 and the last in 1986. Seven machines survive today, one of which is operational. Another one is planned to be refurbished.