Simultaneous Use With Overhead Wire
A railway can be electrified with an overhead wire and a third rail at the same time. This was the case, for example, on the Hamburg S-Bahn between 1940 and 1955. A modern example is Birkenwerder Railway Station near Berlin, which has third rail on both sides and overhead wire. The whole Penn Station complex in New York City is also electrified with both systems. However, such systems have problems with the influence of the different supplies. If one supply is DC and the other AC, an undesired premagnetization of the AC transformers can occur. For this reason, double electrification is usually avoided.
The border station of Modane on the French-Italian Fréjus railway was electrified at both 1,500 V DC third rail for French trains and with overhead wires (initially three-phase, later 3,000 V DC) for Italian trains. When the French part of the line was converted to overhead wires, the voltage of the wires was dropped to 1,500 V DC. Now Italian trains run in Modane feed with 1,500 V DC instead of 3000, with half of their power.
Read more about this topic: Third Rail
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