Multi-system Units
Because of the variety of railway electrification systems, which can vary even within a country, trains often have to pass from one system to another. One way to accomplish this is by changing locomotives at the switching stations. These stations have overhead wires that can be switched from one voltage to another and so the train arrives with one locomotive and then departs with another. Often, however, this is inconvenient and time-consuming. The switching stations have very sophisticated components and they are very expensive.
Another way is to use multi-system locomotives that can operate under several different voltages and current types. In Europe, it is common to use four-system locomotives (1.5 kV DC, 3 kV DC, 15 kV 16⅔ Hz AC, 25 kV, 50 Hz AC). These locomotives do not have to stop when passing from one electrification system to another, the changeover occurring where the train coasts for a short time.
Eurostar trains through the Channel Tunnel are multisystem; a significant part of the route near London is on southern England's 750 V DC third rail system, the route into Brussels is 3000 V DC overhead, while the rest of the route is 25 kV 50 Hz overhead. The need for these trains to use third rail ended upon completion of High Speed 1 in 2007. Southern England has some overhead/third rail dual-system locomotives and multiple units to allow through running between 750 V DC third rail south of London and the 25 kV AC overhead north and east of London.
Electro-diesel locomotives which can operate as an electric locomotive on electrified lines but have an on-board diesel engine for non-electrified sections or sidings have been used in several countries.
Read more about this topic: Railway Electric Traction, Unit Types
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