Towards the end of the 1960s, the Bundesbahn had the first S-Bahn multiple-unit train developed for alternating current networks, which was initially to run as the ET 20 with middle cars designated as the ET 21. The first services were to take place at the 1972 Summer Olympics in Munich. The first trains were given the class numbers 420 and 421 when they were put into service.

Each train consisted of three cars that had no gangways between them. In the middle car there was a first-class compartment, which was later either halved or completely replaced by second-class due to lack of demand. The sets could be coupled to form longer trains with little effort using Scharfenberg couplings. From the start, the middle cars were made of aluminum, and later the end cars were also made of aluminum instead of steel. All axles of the three cars were driven by nose-suspended motors and controlled by thyristors. Braking was usually electric and air brakes were only supplementary.

The production of the total of 480 sets was carried out by several manufacturers and lasted for a period of 28 years. These trains were used on virtually all West German S-Bahn networks that are electrified with the usual AC system via overhead lines and do not have their own DC system via third rail. In addition to Munich, these were also the S-Bahns Rhein-Ruhr, Rhein-Main, Frankfurt and Stuttgart.

The points of criticism of the trains that were repeatedly cited were that there were no gangways between the cars, the individual sets always had to be used with three carsand there were no toilets for longer sections. For this reason, some trains of the Rhine-Ruhr S-Bahn were handed over to Munich as early as 1980.

The class 420 trains were partly replaced by locomotive-hauled trains, but mostly by class 422, 423 and 430 multiple units. Some trains which had already been decommissioned were handed over to other regions when there was additional demand there. Some trains were used from 2002 in the Stockholm area as SL X420 and were scrapped in 2005. In 2006, two trains were upgraded inside and out as “ET 420 Plus”, but no other trains were converted. Although some are still in use today, 400 of the 480 sets have already been scrapped.