Engineering Interfaces Background

Engineering Interfaces - Background

Many of the engineering problems that are of particular importance to railways arise at interfaces (e.g. between vehicles and the track system; the track system and the sub-base/sub-structures; or passengers and vehicles).  Addressing engineering problems in this way avoids the compartmentalisation associated with traditional engineering disciplines and the historically separated infrastructure and vehicle operating responsibiliites within the industry.

This theme will contribute primarily to the areas of reliability, capacity and safety.

Railtrack's 2001 Annual Return to the Rail Regulator stated that 58% of all delays to passengers and 18% of all delays to freight trains were attributable to Railtrack, caused mainly by engineering problems including unplanned maintenance.  It can be argued that new and refurbished engineering systems will need to be more robust, longer lasting and need less maintenance if the required improvements in reliability  and capacity are to be delivered.  The importance of inherently safe vehicles and infrastructure, particularly on heavily trafficked, high speed railways, is self-evident.

The behaviour of the track system is critically dependant on the behaviour of the sub-soil that supports it.  Remediation of track faults (line and level) by ballast tamping can bring only temporary improvement if the real problem is in the underlying sub-base.  Indeed, the major costs of track maintenance come not from movements in the track ballast (which can be dealt with by replacing during short overnight track closures), but from deformation and failure of the formation, which requires lengthy closures of track to allow complete excavation and replacement to take place.  While empirical methods of track system/sub-base design may have been adequate in the past, their relevance to a high speed and heavily trafficked railway is at best uncertain.  It is also increasingly recognised that the future financial investment in railways must be accompanied by advances in infrastructure technology if a succesful, reliable and efficient rail service is to be delivered.  Our current poor understanding of trackbed and subsoil performance, including the lack of a thorough explanation of the mechanisms by which permanent deformation occurs, is a key limiting factor in all of these areas that hinders the development of reliable prediction techniques and the adoption of unambiguous design criteria.

For all of these reasons, the development and application of a better understanding of track/sub-base and track/sub-structure interactions - drawing as appropriate on recent advances in other areas of engineering science such as soil mechanics - forms one of the major foci of the research within Theme A.  This will inform the design of reliable, low-maintenance new and replacement infrastructure including novel track systems, the prediction of maintenance requirements, and the development of better diagnostic tools and novel repair techniques.

A better understanding of wheel/rail interface processes is similarly critical to improving the capacity, reliability and safety of the railway system.  Both reliability and capacity are adversely affected by the imposition of speed limitations due to degraded rails and the interruptions to service caused by rail breaks and maintenance through re-profiling or weld repair.  The safety critical role of the wheel-rail interface - particularly on heavily trafficked, high-speed railway - is well illustrated by the recent accidents at Hatfield, England in 2000 and Eschede, Germany in 1998.

Accurate models of rail life are required in order to implement an efficient, safe rail maintenance and renewal programme.  The problem of excessive rail wear and/or rail breakage is a global one with other countries experiencing different aspects of the same problem and adopting various strategies to manage their rail infrastructure.  Differences in, for example, average axle loads, sleeper spacing and ballast/sub-base result in differences in loading profiles on the rails, while a predominantly single line use (e.g. freight or passengers, high-speed lines etc.) will alter the profile of wear compared to fatigue degradation.  The effects of these differences on rail life and renewal and maintenance strategies can only be reconciled by developing a better understanding of rail technology, in particular rail degradation mechanisms, and the integration of these into rail life models.  This will be of particular benefit in the UK, where the frequency of rail failures is increasing.

The phenomena of rolling contact fatigue (RCF) and wear are reasonably well understood for the rail steels in service and their interaction can be modelled to provide qualitative assessments of rail life.  However, accurate quantitative modelling is not yet possible as there are gaps in understanding and higher quality input data are required.  Fundamental understanding of the degradation modes now needs to be coupled with in-service rail loading and steel microstructural details (including inhomogeneities) to provide improved rail life models.  Thus research in this area focuses on the improvement of fundamental understanding coupled with a greater degree of interdisciplinary work to produce a more complete rail modelling capacity (including loading; microstructure; degradation; type and frequency of maintenance).  Potential practical applications include the prediction of rail life and the development of maintenance and renewal strategies allowing for improvements in rail materials and track system performance  and increases in capacity through heavier, faster and/or more frequent trains.

Noise and vibration are major environmental concerns.  Much of the noise and vibration of the railway system is generated at the wheel/rail interface and requires research of a fundamental engineering nature.  While recent advances have been made in controlling noise by the use of damping devices on the wheel and rail, fundamental aspects of the problem remain.  It is well understood that rolling noise is caused by the surface roughness of the wheel and rail but it is not yet possible to control the growth of this roughness.  This is seen as a major priority for the further control of railway noise.  Thus research in this area will combine metallurgical, tribological, dynamic and acoustic expertise.  Squeal noise generated in sharp curves is also fairly well understood in terms of the modes of vibration of the wheel that are excited, but the major deficiency in current understanding relates to thefriction characteristics at the wheel/rail interface, particularly under unsteady large creepage conditions.

More detailed information will be available to members of the Engineering Interfaces Theme Network