Short pitch corrugation of railway tracks with wooden or concrete sleepers: An enigma solved?

British Railways (BR) in the 1970s observed an increased level of short pitch corrugation after the electrification of the West Coast main line, which involved (among other things) the change from wooden to concrete sleepers. Here, using a simple model for the two systems, we find a different sensitivity to lateral creepage, a doubled growth at the "pinned-pinned" resonance regime shifted from about 600 to about 1000 Hz, but a reduced growth in the 300-600 Hz range, where most data for both systems seem to lye. Hence, despite it would be tempting to associate the increased corrugation simply to the increased (doubled) peak of normal load, there is discrepancy in the corresponding wavelength predicted by the model. Hence, there is still an "enigma" about the reasons for the enhanced corrugations