Shortly before the turn of the millennium, Deutsche Bahn and SNCF started a cooperation to develop a modern, single car diesel multiple unit. Partner companies were the French De Dietrich Ferroviaire and German Linke-Hofmann-Busch, both of which now belong to the French Alstom group. Thus, the resulting vehicle was subsequently added to the Alstrom Coradia family, while the “TER” stands for “Transport express régional”, which is used in France for fast regional trains. While in France they are also known by the nicknames „cucumber”, “suppository” and “sausage” because of their shape, in both countries they are usually called “whale” or “blue whale”.

The special thing about the vehicles is that the engines are located at both ends in front of the bogies in the floor and result in a massive appearance. The power is transmitted to the inner axle of the adjacent bogie via a Voith hydrodynamic transmission. This structure allows for a floor height of 55 cm in the middle part, which is positive when used at rural stations without a high platform.

The vehicles were built in both countries of origin, 40 of them in Germany. These are operated by DB Regio as class 641 and are primarily used on non-electrified branch lines. By far the most common variant is the X 73500 of the SNCF, which has a top speed of 140 km/h and of which 312 examples were built. There are also 19 X 73900s for cross-border traffic to the Saarland, which can be used in multiple traction with the German vehicles, and six units from the 2100 series of the Luxembourg CFL.

In France, the vehicles repeatedly make negative headlines because they have been involved in accidents at level crossings with barriers failing to close and in incidents when shunting. This was attributed to the fact that the vehicles often do not trigger track circuits for switching barriers or signals because they either have too few axles or, due to their low weight, do not sufficiently displace dirt on the tracks and thus prevent contact closure. As a countermeasure, they are often used in multiple, even if the number of passengers does not require this.