To accelerate traffic on the mountain line, the BLS ordered a development of their Ae 4/4 in the early sixties. It was developed by SLM and BBC and with an output of nearly 5,000 kW, it was one of the world's most powerful four-axle locomotives of its time. Originally it had a top speed of 125 km/h and was designated Ae 4/4II since this speed qualified it for the Swiss train type A. Along with the development of the similar SBB Re 4/4II, changes were made in the suspension and the authorities could be convinced that even locomotives with an axle load of 20 tonnes could be approved for 140 km/h. So both locomotives were approved for the higher speed and qualified as train type R, what led to the renaming of the Ae 4/4II into Re 4/4.
A total of 35 locomotives were built in several batches between 1964 and 1983. In the beginning, they were only used in express service. With a train of 630 tonnes, they could reach 80 km/h on the 2.6 percent ramp. Later they were used with all train types. After the Re 465 was introduced in the nineties, the Re 4/4, now called Re 425, were also equipped to run in multiple with the Re 465. Since 2014, only a few Re 425 are being used in passenger service. Freight services are now also being taken over by the Re 485.
