Changes of perceived unevenness caused by in-track vibration countermeasures in slab track

Abstract

This paper investigates the relation between unevenness at different locations in a railway track and the unevenness perceived by a train, for the particular case of a slab track. The different types of unevenness considered are: slab and track bed unevenness, bent rail and bent slab, variation of rail pad stiffness, variation of slab bearing stiffness, and variation of rail and slab bending stiffness. The unevenness perceived by the wheels of a moving train is the result of all these types of unevenness. When vibration countermeasures, such as resilient elements (e.g. soft rail pads), are introduced in a track, the perceived unevenness is modified. The slab track is modelled as a double beam supported on a rigid foundation. Analytical expressions are derived to relate the input and perceived unevenness. It is shown that in-track vibration countermeasures always introduce a change in perceived unevenness, which may turn out to be an increase or decrease and, consequently, result in a change of the performance of vibration countermeasures. This is illustrated for the particular case of a slab track on elastic supports on metro line M1 of ATM in Milan (Italy). This paper shows that prediction of vibration insertion loss based on conventional models may lead to inaccuracies in absence of information about different sources of unevenness. After the installation of vibration countermeasures, it is advised to measure the perceived unevenness in order to interpret differences between predicted and measured insertion loss.