The Class 423 is an Electric Multiple Unit (EMU) made for the S-Bahn commuter networks of Germany (Munich S-Bahn, Cologne S-Bahn, Frankfurt S-Bahn and Stuttgart S-Bahn). It is similar in size to the Class 420 DMU (which is its predecessor) but benefits from being lighter and from having one large passenger compartment as opposed to that of the Class 420 which is divided into three.

A Class 423 EMU comprises four cars which share three jacobs bogies, and the cars can only be separated at maintenance facilities. The two driving cars are each designated Class 423 and the two inner cars are designated Class 433. The Class 423 EMU typically runs as either the four-car set, or as two four-car sets coupled together.