Résumé
This paper illustrates the phenomenon of the critical speed in the range 100- 300 km/h. A real explanation and a precise calculation of the critical speed seems to rest an unresolved problem in many cases. Some authors, analogous to "Mach effect", have attributed this phenomenon to the reaching the Rayleigh wave speed of substructure by trains. But this explanation can't be thorough in many cases because the Rayleigh wave speed is very high and the phenomenon happens even for lower speeds. This work shows an evident relationship between the receptance of the train/track coupling system and the so called sleeper-passing frequency. It is shown that the amplification of vibrations happens when the train speed causes the equivalence between the sleeper-passing frequency and the receptance of the train/track coupling system. This explanation seems to be definitively complete even if only a numeric validation is presented in the paper.