Ageing Cuts Down the Track Homogeneity Causing Differences Between Calculations and Measurements of Railway Noise

Abstract

At tracks with high accumulated axle loads per year the ageing effects of the rail pads will change track parameters causing different noise radiation than calculated by general data sets. This observation initiated the R&D-project Factor X, funded by Schweizerischen Bundesbahnen (SBB). The changes due to ageing effects act in different ways on the overall system. First effect of the axle loads acting on the rail pads will lead to a loss in pad thickness over time with the consequence of a decreased clamp forces at the rail foot. This reduced clamping force allows a higher grade of rail movement presented as increased local mobility of the rail head, leading to increased noise radiation. The second observed ageing effect was the loss of stiffness of the rail pads under the impact of accumulated axle loads. This effect leads to a decoupling of the rail from the sleeper, again leading to increased noise radiation. With an increased accumulated axle load the spread of the ageing effect of the rail pads becomes more stochastic. A consistent prediction method about the changes of parameters is impossible to be given. A well-proven method of enhancing the basic data set for calculation is achieved by measuring and calculating the TDR of the track under investigation. As the calculation method of the TDR data mainly depends on the local mobility of the rail at the starting position of a measurement grid, the uncertainty of the calculated TDR also depends on variations from sleeper bay to sleeper bay due to the wear state of the rail pads. This uncertainty of calculated TDR data can be reduced efficiently by increasing the number of measurement grids along a track section, allowing an analysis of the deviation over several measurements. Due to the duration of one measurement grid containing 29 positions, a new measurement method with a reduced number of positions in a grid is presented. By a choice of appropriate positions the loss in precision of calculating TDR is almost negligible.