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The necessarily single-line transfer of electric energy to the moving railway vehicle prohibited the use of the simple three-phase induction motor in traction, until power-electronic converters were mature. The single-phase series-wound commutator motor which had to be used in AC mainline electrification instead enforced a low system frequency, 162/3 Hz in Central Europe, introduced exactly 100 years ago. This had the consequence of a proprietary system of generation and high-voltage transport, separate from the public three-phase mains. Since 1990, power-electronic converters gradually took over the task of generation of the 162/3-Hz current and will replace former single-phase generators and rotary converters. For railways with direct 50-Hz feeding, which have been introduced after 1950, power-electronic converters promise a distinctive improvement, as abolishing the hindering phase insulations in the overhead lines, which enabled to distribute the single-phase traction loads more or less evenly to the three-phase public grid.