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AC circuit breakers are easier to design for high voltages, removing this limitation and enabling more power. The electrification pioneers knew this, & wanted AC OLE - but that meant on-train rectification. A few pioneer railways successfully ran AC systems – in the UK the Lancaster - Heysham (1908) and London Victoria - London Bridge (1909) systems both ran at 6.7kV 25Hz AC. But the reliability of Series DC motor control meant AC traction did not make headway in the UK until after World War 2.