High speed railway track dynamic behavior near critical speed

This study was performed on the Amtrak Northeast Corridor (NEC) at Kingston, Rhode Island where is known as the Great Swamp and requires more frequent track maintenance. It was suspected that the so-called "critical speed" condition might exist at this particular location. The critical speed is the speed at which trains travel on the soft subgrade close to or higher than the Rayleigh wave velocity of the subgrade soil. The conventional understanding of the "critical speed" would expect both a cone-shaped ground wave motion and substantial amount of track deflections. Field investigations combined with a validated 3-D dynamic track-subgrade interaction model were used to evaluate the track performance and determine if the critical speed effect exists at the Kingston site. The track performance was investigated by a three-by-three (3 x 3) array of accelerometers. Site investigations were carried out to characterize the site and provide input data for modeling. According to the field measurements and model results, the rail did not show excessive deflections; however, ground surface wave propagation had been detected with a cone-shaped mode. In other words, the cone-shaped ground wave motion and the increase in rail deflection did not occur at the same time as the conventional understanding. In addition, the model results pointed out that the stress level in the subgrade would encounter a significant increase under the current operational speeds (less than 250 km/h) rather than excessive rail deflections and the rail deflections will increase dramatically at the simulated train speeds of over 300 km/h. Therefore, the "critical speed" is defined in two levels for the Kingston site: 1) The speed causing significant stress increase in the ballast and subgrade, at which more frequent ballast maintenance is needed; 2) The speed causing significant increase in rail deflection, at which derailment becomes a concern.