Railway noise – in this context, rolling noise – is the result of complex interactions between the vehicle and the track. The roughness of the two contact partners – the running surfaces of the wheel and the rail – result in non-stationary excitation forces which act both on the vehicle and the track.These excitation forces generate a structure-borne sound field in the rail and the wheel. This structure-borne sound field is radiated as audible airborne sound that nearby residents may perceive as noise. The vibrations in the railway superstructure caused by the excitation force may also damage the components of the superstructure or lead to accelerated ageing. Moreover, there is a risk that the energy could be propagated into the ground and become a source of vibrations. On the vehicle side, for example, these non-stationary excitation forces are taken into account by the axle suspension. In the case of the superstructure, elastic elements such as the intermediate pad or sleeper padding serve to diminish the force peaks or reduce the transferred energy.
Products are being developed to counteract noise radiation, ageing of components and vibration emissions. This is achieved, for example, by inserting a soft intermediate pad between the rail and the sleeper. The structural-dynamic decoupling of the rail is thus increased, and less energy is transferred into the subjacent components. As the result of this measure, sleepers and ballast experience a reduced dynamic load, which positively impacts their lifetimes. It has nevertheless been proven that this causes more energy to be radiated into the rails and then emitted as potentially disturbing airborne sound. It is therefore apparent that the optimum in one specific area (lifetime) can lead to problems in another specific area (noise). Consequently, the ongoing development and optimisation of the railway infrastructure by means of newly developed components requires an approach based on a consideration of the overall superstructure-vehicle system: Lifecycle costs, noise reduction, safety, operations, and other factors must all be considered.
Realistically speaking, holistic optimisation of the railway superstructure is only possible with the use of physics-based models. Empirical tests would involve too much effort and cost. The Railway Field Laboratory continuously supplies data that is of importance for the physical understanding of the superstructure. Thus, for example, changes in the measurement data are documented in relation to the type of vehicle passing over the track and the ambient meteorological conditions, and over time (ageing effects); these changes can be used as input for the construction of models and assist in their validation.
Because numerical models cannot comprehensively map the real world, a new or optimised component must be tested in real operation. Experience shows that critical issues often arise when such trials are undertaken. Only in the rarest cases is the comparability of the data collected during a trial admissible without further processing, because the dynamic behaviour of the overall system depends on factors such as environmental conditions (e.g., temperature, air humidity, etc.), location (e.g., ground conditions) and ageing effects (e.g. new versus old pad), etc. In addition, the design of the experiment (corresponding sensor positions, data acquisition and evaluation, etc.) influences the comparability of the collected data. Many of these factors are not recorded (or are only recorded incompletely) during tests and are difficult to monitor.
Solutions for many of the problems addressed in the foregoing sections are made possible by the Railway Field Laboratory operated by the Federal Office for the Environment (FOEN) together with the Swiss Federal Office of Transport (FOT), Swiss Federal Railways (SBB), the Swiss Federal Laboratories for Materials Science and Technology (Empa) and the Allianz Fahrweg ("Swiss Permanent Way Alliance"). In the Railway Field Laboratory – which, to a certain extent, is a reference track in real operation but with meticulous and continuous recording of all influencing parameters – changes can be observed over time, or the influence of environmental conditions on the overall system can be monitored. The effects of modifications to individual components can be investigated and understood because the structural-dynamic baseline is recorded and known. These statistically validated statements are made possible thanks to the permanent basic equipment of the test track, consisting of sensor technology, data acquisition and a data management system. As well as guaranteeing the comparability of the trials, this ensures that consideration is given to environmental and ageing influences on the data collected.