railroads. The Key System's first trains were composed of standard wooden railroad passenger cars, complete with clerestory roofs. Atop each of these, a pair of pantographs, invented and manufactured by the Key System's own shops, were installed to collect current from overhead wires to power a pair of electric motors on each car, one on each truck (bogie). The design of rolling stock changed over the years. Wood gave way to steel, and, instead of doors at each end, center doors were adopted. The later rolling stock consisted of specially designed "bridge units" for use on the new bridge, articulated cars sharing a common central truck and including central passenger entries in each car, a forerunner of the design of most light rail vehicles today. Several of these pairs were connected to make up a train. Power pickup was via pantograph from overhead catenary wires, except on the Bay Bridge where a third rail pickup was used. The Key's trains ran on 600 volt direct current, compared to the 1200 volts used by the SP commuter trains. The cars had an enclosed operator's cab in the right front, with passenger seats extending to the very front of the vehicle, a favorite seat for many children, with dramatic views of the tracks ahead. The exterior color of the cars was orange and silver. Interior upholstery was woven reed seat covers in one of the articulated sections, and leather in the other, the smoking section. The flooring was linoleum. During WWII, the roofs were painted gray for aerial camouflage. After acquisition by National City Lines, all Key vehicles including the bridge units were re-painted in that company's standard colors, yellow and green. Transbay rail lines Until the Bay Bridge railway began operation, Key commuter trains had no letter designation. They were named for the principal street or district they served. D was reserved for a proposed line into Montclair alongside the Sacramento Northern interurban railway The A, B, C, E and F lines were the last Key System rail lines. Train service ended on April 20, 1958, replaced by buses using the same letter designations. AC Transit preserved the letter-designated routes when it took over the Key System two years later, and are still in use; AC Transit's B, C, E, F, G and H lines follow roughly the corresponding Key routes and neighborhoods. East Bay Street railways The Key System's streetcars