The class 423 was the first in a series of S-Bahn multiple units that had been purchased since the late 1990s to replace the aging class 420. Although the latter had been built up to that point, it was a 1960's design and the first examples had already reached the end of their useful life.
In contrast to its predecessor, the class 423 consists of four instead of three cars with the same overall length. While each car previously had two bogies of its own, the middle cars now rest on shared Jakobs bogies. This is just one of the weight reduction measures used, which reduced the curb weight by a total of 24 tonnes with the same number of seats. With 50 kW less motor power, acceleration was also significantly improved compared to the newer, weight-optimized versions of the 420 series. Four of the five bogies have two powered axles. The gangways between the cars are now open, which, among other things, enables a better distribution of passengers. With now three instead of four double doors per side, the total number of doors remained the same to enable quick entry and exit.
The vehicles quickly got the nickname “Quietschie” (from the German word for “squeak”), which came about due to a special feature of the combined power and brake lever. When starting off quickly, this is responsible for brakes still being applied lightly when the power is already switched on and therefore make a squeaking noise.
The classes 424 and 425 followed in 1999, which are identical to the class 423 in terms of length and power train. The different classes can be explained by minor adjustments for different areas of application. The class 425 has a toilet, fewer doors and different entry heights so that it can also be used outside the actual S-Bahn network. Some classes have LZB and are allowed to run at 160 km/h. The class 426 is a two-car version of the 425 with two of the three bogies powered. This is used for routes with low passenger volume and to reinforce existing trains. The most modern variant is the class 422, which has been built for the Rhine-Ruhr S-Bahn since 2007. The most important change is the adaptation of the vehicle fronts to the latest crash standards, which has increased the overall length by around two meters with additional crumple zones and anti-climbing devices.
There were some delays in the certification, which were caused, among other things, by a lack of braking performance in bad weather, software problems and problems with the door locking systems. By 2010, over 600 trains had been completed, of which 462 belonged to the class 423. Since 2013, some trains have been optically adapted to the even more modern class 430, which, in addition to LED lighting and an expanded passenger information system, also includes improvements in accessibility.