In 1958, the SEAG company built two prototypes of the BTmms 58 derived from the BTms 55, which was designed for four "pa" containers. The cars, one built of light metal and the other of ST 52 steel, could now be loaded with five "pa" containers. This allowed the specific cargo weight to be increased, which in turn allowed the carrier to pay more advantageous rates for freight. The prototype made of steel proved its worth and was further developed to prepare it for volume production, which began in 1960. Over the next six years, a total of 2100 cars of the BTmms 58 class were produced. To simplify dispatching and loading, one end each car had a transition platform located above the buffers or a hand-brake platform in front of the vehicle frame. DB simultaneously exchanged the 35-tonne buffers for heavy duty 59-tonne buffers which were also used from the outset on cars constructed from 1964 onwards. From the end of the 1970s, a majority of the vehicles were prepared to accommodate the AK69e automatic central buffer couplings, but these were not installed in the end. As the further development of the Ekrt 211 closed medium container, the Ekrt 212 is distinguished by its smooth exterior. The necessary rigidity was achieved by means of vertical reinforcing seams on the side surfaces. To simplify loading and unloading, the Ekrt 212 had double wing doors that reached over the entire container height. The total of 200 containers built from 1958 were a favourite with the customers of Deutsche Bundesbahn, because the smooth outer surfaces were especially suitable as advertising surfaces.