To complement or replace the Ae 6/6 on the Gotthard, the SBB introduced the Re 6/6 in 1972. Its tasks were not only to haul freight trains on the Gotthard ramp, but also heavy passenger trains had to be handled without the help of another locomotive. The prefix “Re” indicates the higher top speed of 140 km/h, what means that it could also be used for passenger trains on level lines.

For better running characteristics and less wear on curvy lines, they got three two-axle bogies of which the central one has a large amount of lateral flexibility. Only two of the four prototypes had been built with a one-piece body, while the other two were split into two articulated parts like the RhB Ge 6/6^II. In the end, the conventional one-piece variant was used. The electric part was similar to that of the Re 4/4^II, but its hourly output of 7,850 kW (10,527 hp) was more than 1.5 times as high.

After the prototypes built in 1972, 85 more were built between 1975 and 1980. They were often used in double with a Re 4/4^II, and this combination was also referred to as Re 10/10. They can haul up to 1,400 tonnes on the 2.6 percent of the Gotthard, and with an additional locomotive at the end this can be increased to 1,700 tonnes. With the new class scheme of 1992, the Re 6/6 became the Re 620. Since 2003, all are being owned by SBB Cargo.