For the 30,000 km long French conventional railway lines (94% of the whole network), the train speed is currently limited to 220 km/h, whilst the speed is 320 km/h for the 1800 km long high-speed lines. Nowadays, there is a growing need to improve the services by increasing the speed limit for the conventional lines. This paper aims at studying the influence of train speed on the mechanical behaviours of track-bed materials based on field monitoring data. Emphasis is put on the behaviours of interlayer and subgrade soils. The selected experimental site is located in Vierzon, France. Several sensors including accelerometers and soil pressure gauges were installed at different depths. The vertical strains of different layers can be obtained by integrating the records of accelerometers installed at different track-bed depths. The experimentation was carried out using an intercity test train running at different speeds from 60 km/h to 200 km/h. This test train was composed of a locomotive (22.5 Mg/axle) and 7 “Corail” coaches (10.5 Mg/axle). It was observed that when the train speed was raised, the loadings transmitted to the track-bed increased. Moreover, the response of the track-bed materials was amplified by the speed rise at different depths: the vertical dynamic stress was increased by about 10% when the train speed was raised from 60 km/h to 200 km/h for the locomotive loading, and the vertical strains doubled their quasi-static values in the shallow layers. Moreover, the stress–strain paths were estimated using the vertical stress and strain for each train speed. These loading paths allowed the resilient modulus Mr to be determined. It was found that the resilient modulus (Mr) was decreased by about 10% when the train speed was increased from 100 km/h to 200 km/h. However, the damping ratio (Dr) kept stable in the range of speeds explored.