The rolling stock on the Berlin U-Bahn are the main types of cars for the underground railway (subway). They are split into two general categories: Kleinprofil ("small profile", used by the U1, U2, U3 and U4) and Großprofil ("large profile", used by the U5, U6, U7, U8 and U9) lines. The names refer to the size of the train's coaches. Großprofil coaches have a width of and a height of metres, and Kleinprofil coaches are only wide and high. Therefore, the trains have to operate on separate networks. Both networks have track and are electrified at 750 volts DC. Because Großprofil and Kleinprofil trains use different types of power supply the trains cannot normally operate on the same route. However, on the Nord-Süd-Bahn in the years between 1923 and 1927 and on the E line (today's U5) between 1961 and 1978, Kleinprofil trains with specially adapted power pickups ran on Grossprofil tracks. They were fitted with special wooden boards on the sides to close the gap between platform and train. These wooden boards were jokingly called Blumenbretter ("flower boards"). The polarity of the power rails also differs. On the Kleinprofil lines the power rail is positively charged and the track is negative, on the Großprofil lines it is the other way around. In East Berlin the polarity of the track section Thälmannplatz/Otto-Grotewohl-Straße - Pankow, was the same as on the Großprofil lines. After reunification, this exception to the normal Kleinprofil polarity was reversed by the BVG, even though there are benefits to this arrangement (there is less corrosion of metal parts in the tunnel with the Großprofil polarity). The newest types of U-Bahn are H for the Großprofil and Ik for the Kleinprofil. The oldest vehicles still in service are of the F74 type (Großprofil) and the A3E type (Kleinprofil). Train profiles Kleinprofil Today, only trains of the Ik, Hk, GI/1E, A3E and A3L71-A3L92 types are in active service. A type cars Two test vehicles were ordered for the first Berlin U-Bahn line from the Cologne coach builders, van der Zypen & Charlier. One of these vehicles was used by Wilhelm II in 1908, leading to their nickname Kaiserwagen ("emperor's coach"). The train width of was already fixed at this point. At that time, trains and subways were still modelled on streetcars. The first production vehicles, which were appropriately titled A-I, were built in the Warschauer Brücke workshop. At the