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Many single-phase 25 kV electrified railway systems carry a large number of conventional thyristor-based locomotives, which draw current rich in harmonic content. A possible solution to mitigate this harmonic complication is to connect a power quality conditioner based on voltage detection at the end of the traction feeder. However, the performance of the installed power quality conditioner depends strongly on the accuracy and stability of the harmonic extraction algorithm used to generate the command harmonic current reference to be tracked by the inverter inner current control loop. A high performance, computationally efficient harmonic extraction algorithm is presented, implemented using delta-operator-based digital resonant filters for traction applications. Unlike most other extraction algorithms, the implementation of the algorithm presented does not require complex mathematical transformations, and therefore has a significantly faster computation speed and is suitable for single-phase traction applications. Also, through delta-operator-based implementation, the proposed algorithm exhibits improved accuracy and robustness over a wider range of operating conditions. The accuracy and robustness of the proposed algorithm have been verified by both simulation and experiment.