In recent years, there has been a global trend towards increasing the commercial speed of high-speed rail lines (HSLs) to cope with expected demand problems. Examples include the new HSLs projects for the UK and California, where the design speed reaches 400 km/h. Therefore, one of the biggest challenges currently faced by the railway sector is the determination of the critical speed value of railway sections. This is true both for future HSLs, due to their high design speeds, and for existing rail lines, due to the intended increase in commercial speed. The critical speed is that train speed that produces a dynamic amplification in the medium underlying the track, causing an amplification of the vertical movement of the track components and the supporting ground such that the stability of the infrastructure and the safety of passengers is compromised. This work sets out the methodology necessary to experimentally study the critical speed of a railway section by in situ applying surface wave spectral analysis techniques, SASW and MASW. The critical speed is determined by the lowest phase velocity of the local minima of the modal dispersion curves, or by the minimum of the apparent dispersion curve. Both methods are equivalent. This paper also presents the results obtained on some Spanish HSLs, both on tracks under construction and in operation. In the cases studied, the ballast layer is the one that presents the lowest shear wave velocity and the minimum of the dispersion curve, so it is the layer that determines the critical speed.