As the Deutsche Reichsbahn suffered from a major shortage of wagons at the beginning of the 1920s, new passenger coaches of standardised design were procured from 1921 onwards.These were all to be designed with an iron body, but due to production problems most were delivered with a wooden body.  By 1930, more than 8250 of these wagons had entered service. The wagons were nicknamed  “thunder box” because the steel body tended to drone heavily. The name was then also applied to the wooden version of the wagons. After the war, some Bi cars were converted by the DB into driving trailers for steam locomotive-hauled push-pull trains.

Our three coaches (84 803, 84 865, 85 466) were the last three in the Deutsche Bundesbahn stock and were bought in 1976.