N/n/n1 and N1/n2 stock
When the city of Vienna took over the Stadtbahn operation from the Federal Railways in 1925, the contract was limited to a mere 30 years, and the city did not want to invest too much money in a system that they did not really own. Hence, the rolling stock bought for operation on the metro had to be of a type that could also be used as tram cars; due to the steep gradients, pneumatic brakes were fitted. This stock, which consisted of N motor cars, n heavy trailers and n1 light trailers, was used for almost 40 years. By then, they were barely operational.
N/n1 stock (Alfred Luft)
N1/n2 stock and N/n1 stock (H. Hermann)
Although at that time articulated tram cars had come into use on Vienna's tram system, the decision for the metro was to merely convert the old N and n1 cars into new N1 and n2 cars, even though the city already owned the metro by then. Whereas the car bodies were completely replaced, the motors were taken over from the old stock and the bogies were only slightly modified. These trains were operated in a variety of formations, from N1-n2-N1 three-car trains to N1-n2-n2-n2-N1-n2-n2-n2-N1 nine-car trains. N1/n2 stock remained in service until 1983.