lines supplying 6,600 V alternating current at 25 Hz. The electricity came from a coal-fired power station in Leverkusenstraße in Bahrenfeld, which also provided power to the Altona harbour railway. The first electric trains ran on 1 October 1907, and from 29 January 1908 the line from Blankenese to Ohlsdorf was served exclusively by electric trains. These dates are considered the birthdates of the S-Bahn. The basic unit of an AC train consisted of two articulated compartment carriages on six axles with motorised two-axle bogies under each cab end and a Jacobs bogie in the middle. The carriages, with doors on each side of the compartments, took their design and functionality from Prussian compartment carriages. 1924: Network expansion along the Alster valley railway A railway line constructed and operated by a local company in 1914 and taken over by the district of Stormarn after bankruptcy, led from Ohlsdorf to Poppenbüttel in Prussia, with the goal of connecting neighbouring settlements along the Alster river. It was known as the Alster Valley Railway. It opened in 1918 with petrol-powered trains. The district of Stormarn gave the line to the German Imperial railway company, which electrified it and provided the extension of the Hamburg-Altona City and Suburban railway to Poppenbüttel in 1924. 1934: Designation as an S-Bahn The term S-Bahn was first coined and used in Berlin from 1930, where a similar system on the City, Ring and Suburban lines had been operated since 1924; it was first used by the German Imperial railway with reference to its Hamburg-Altona City and Suburban railway in 1934. The term was also used to describe non-electric services on lines within the local suburban tariff: the steam-powered lines from Blankenese to Wedel, from Altona to Elmshorn, from Hamburg to Friedrichsruh and to Harburg. Since 2002, lines not served by S-Bahn have been designated "Regional railway lines". 1939/40: Moving to the DC system In the 1930s, after almost 30 years of service, the necessity to renew the trains and infrastructure had become apparent. Since the DC system had proved itself over more than a decade with the Berlin S-Bahn, where the 750 volt DC power was supplied by a third rail, the German Imperial Railway decided to adopt the same system for Hamburg in 1937 and to abandon overhead AC. In order to allow improved acceleration, the S-Bahn uses 1200 volts: as a consequence, Berlin and Hamburg rolling