A non-negligible amount of active power is carried also by harmonic (or, in general, spectral) terms, especially for highly distorted systems, such as electrified ac railways. The paper considers the distribution and correlation of these spectral active power terms, including train operating conditions, and gives criteria to select and group harmonic power terms. Several processing and display techniques are used to this aim: scatter histograms for joint probability, correlation coefficients, V-I maps. Results are shown and discussed for the Swiss 15 kV 16.7 Hz railway, using measured pantograph voltage and current quantities.