The designation X31K stands for three-car electric multiple units originally ordered by the SJ and designed for the traffic between Sweden and Denmark, across the Öresund. This required that they had to be capable for both the 15 kV power system in use in Sweden and the 25 kV power system in use in Denmark. Built by Adtranz which later became Bombardier, they are being called “Crusalis Contessa”. The DSB calls these trains ET.

Both end cars of each train are being powered on all four axles and provide a top speed of 180 km/h. The central car offers a low-floor section and houses the pantographs and transformer. Like some multiple units in use in Denmark, they also have inflatable rubber noses which include a fold-away driving desk. Up to five units can be operated in multiple, creating a 15-car train that has 1,145 seats and allows the passengers to move freely through the train.

A total of 104 sets was built between 1999 and 2012. As planned, these trains are mostly being used for services across the Öresund. About half of them is being owned by the DSB and the others by Öresund AB. Eleven more have been ordered by the Svenska Länstrafikbolag (Swedish rural traffic company). Starting in 2017, they were modernized in the interior.