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This paper investigates the relationship between train speed and acceleration amplitude measured through axle‑box acceleration (ABA) at insulated rail joints (IRJs) on the Norwegian railway network, using the track‑recording vehicle Roger 1000. A total of 56 passages (from two IRJs) recorded with a tri‑axial piezoelectric accelerometer mounted near the axle box on the leading wheelset were analyzed. For each passage, a +-2.5 s window around the IRJ was extracted to identify the exact peak where the train is assumed to have passed the IRJ, and a RMS-value of the absolute amplitudes was computed in a +-1 ms gliding window to show the local vibration energy. Second-order regression models were fitted for both vertical and longitudinal directions that show a near-linear increase in amplitude with speed, with the vertical direction exhibiting the highest sensitivity. Further, normalized RMS-values with speed based on the second order regressions help reveal outliers from a perfect amplitude-speed-relationship, indicating additional influences. The longitudinal (X) direction exhibits larger relative variability than the vertical response after speed normalization, and the changes following documented rail-grinding events were more pronounced in X-direction, suggesting that longitudinal accelerations are more sensitive to subtle changes in track condition.