The testing of rolling stock for the assessment of disturbance in signalling circuits is considered with a focus on the measurement process and the selection of operating conditions. The definition of interference limits is briefly reviewed, but they are considered an external input. The spectral behavior of acquired signals and the evaluation of repeatability are instead discussed with the help of three different real test cases, considering correspondence between similar operating points and differences as rolling stock operation evolves during a test run. Repeatability evaluated as standard deviation is in the order of 3% to 5% in the harmonic and audio frequency range for the different systems, slightly lower for AC railways; above about 10–15 kHz it increases to 30%. Uncorrelated components (with much lower amplitudes) may show much higher dispersion.