TRACK alignment quality is defined by geometric limits and is currently checked using corresponding geometric measurements. However, these measurements have their limitations, particularly in the high-speed sector where very small deviations with long wavelengths can result in significant vehicle reactions. Measuring these deviations using geometric measurement systems also remains a difficult process, while the test runs required for this process decrease line capacity.
Track quality is not the only factor to affect vehicle behaviour; the condition of the vehicle itself and its speed are other important contributing elements. Indeed the recent Dynotrain research project showed that no linear relationship exists between track alignment deviation and the resulting behaviour of the vehicle, which implies that the amplitude of the track alignment deviation as the sole criteria for safe guidance, as described in EN 14363, is not sufficient.
As a result, the question arises of whether a direct measurement of the reaction forces at the wheel/rail interface can assess and monitor track alignment quality.
By directly measuring wheel/rail forces with measuring wheelsets, it is possible to directly assess safety measures that can prevent derailment, track lateral forces, and the single forces which could be damaging the rails. However, this method has its limitations.
As a consequence two methods to assess track quality using measuring wheelsets are conceived:
• installing the measuring wheelsets on a service train so that its reactions can be directly used to assess the quality of the track, and
• the measurements are recorded using a diagnostic vehicle, which is equipped with measuring wheelsets - in this case, a reference profile of wheel/rail forces is recorded on a rule-compliant railway line and changes in these reference profiles are analysed.
In addition to conventional geometric measurement, German Rail (DB) is already using measuring wheelset technology on a diagnostics vehicle and an ICE-S test train and a class 612 DMU. These trains are acting as the reference for high-speed lines and those used for tilting operation in Germany, with tests carried out using the two trains identifying certain track errors not found by the geometric track alignment measurement using the wheel/rail force measurement method.
Despite the clear advantages of this system, which has increased track quality at critical junctures in Germany and several other countries, this method has not yet been standardised and as a result is only used by a handful of infrastructure managers.
One reason for this is the complexity of measuring wheelset systems, which require operation by an expert in the field. In addition the axial signal transmission system has the disadvantage of requiring a cross-hole to be drilled into the wheelset axle. This hole reduces the axle's resistance to fatigue and increases required ultrasonic testing intervals compared with conventional axles.
Another major concern is the increased cost of using measuring wheelsets. The additional benefits on high-speed or lines used by tilting trains are clear. However, on conventional lines the cost benefits are countered by the shorter maintenance intervals warranted by the axial signal transmission system.
To address these concerns and to increase the attractiveness of measuring wheelsets, Prose has introduced an enhancement. Instead of the axial signal transmission, it has developed an inductive-based radial system, which is contactless and as a result almost maintenance-free.
While this solution appears to pave the way for the complete adoption of measuring wheelset systems on standard rail vehicles that would provide continuous infrastructure diagnosis, a number of challenges remain:
• Localisation. If the infrastructure diagnosis is performed with measuring wheelsets, a precise position on the line must be mapped with the measurement results.
• Operation. A measuring wheelset system should be straightforward to operate without supervision when used in a conventional train, and be integrated directly into a diagnostics vehicle's systems.
Measuring wheelsets incorporated into diagnostic vehicles serve as an extension of geometric measurement systems and should be operated by well-trained staff with access to detailed data and results from the tests. In addition multiple options to operate the system must be available.
Prose's measuring wheelset computer achieves this by calculating the wheel/rail forces and providing the information for the user either in a clear and easy-to-use browser interface or in a machine-operable web-service interface. This interface relies on TCP/IP and HTTP and allows a third-party system, in this case the diagnostic vehicle, to operate the measuring wheelset computer. As a result the user does not perceive the measuring wheelset system as a different measuring system by the user. It becomes an integral part of the diagnostics vehicle.
Integrating the measuring wheelset system into a standard rail vehicle is, however, a significantly more complex undertaking. One critical requirement is that the system does not affect normal operations. For example the failure of a sensor must not have any impact on operations or require an intervention by a member of train staff. Accordingly, the system must be able to perform the following tasks independently:
• determine wheel/rail forces
• map data to a location, and
• inform the control centre about any possible exceedance.
In addition, the control centre must be able to download any exceedance data for detailed review, while it should be possible to conduct all other operations of the measuring wheelset system directly from the control centre.
In this scenario Prose's web-based service comes into its own. Originally developed for remote server access, the internet-based technologies are ideally suited to access a system in a moving train, assuming that the train has an internet connection.
The problem with localising the measured data from the railway network is solvable by using a combination of the information available on the vehicle, such as its location and speed with external signals such as GPS.
Monitoring track quality by measuring the wheel/rail forces has specific advantages compared with the geometric inspection. In particular the greater proximity to the target of "secure track guidance," as well as the capture of long-wave track failures on high-speed lines is beneficial.
By simplifying the measuring wheelset system, Prose has improved the economics of this process potentially opening up enhanced track monitoring to more infrastructure managers, allowing targeted improvements to track alignment quality.