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Graphic element Research > Growth > Research projects > Land & marine transport projects> Let the train take the strain - quietly
Graphic element Let the train take the strain - quietly

In recent years, the issue of transport noise has become a high profile issue and the industry has come under pressure to reduce operating noise. While much has been achieved, the European Commission has become concerned that the rail sector has not shown the same progress towards quieter operation that rival transport modes have made. A Green Paper from the Environment Directorate in 1996 underlined this concern, and since then the Commission has contributed towards the funding of research programmes aimed at cutting railway noise - especially in the movement of freight where improvements have been least pronounced.

Fifty years ago people living near railway lines had to put up with substantial noise. The exhaust of steam locomotives, wheels hitting rail joints, brakes squealing and clanking wagon couplings were all part of the background noise. Modern railway operation has dispensed with many of these sources of noise. Quieter diesel and electric traction has replaced steam.

Continuously welded rails provide a smoother ride, eliminating the 'clackety clack' of wheels hitting rail joints. The close coupling of vehicles has done away with the clanking of buffers. On passenger coaches, disc brakes make vehicles much quieter than old tread-brake designs. However, substantial sources of noise remain - particularly from freight vehicles, where brake blocks operating directly on the wheel treads remain in widespread use.

In modern railway operations most noise comes from the interaction between the steel wheel and the steel rail, so the research has been directed primarily at this area. On freight vehicles the problem is compounded by the continuing use of tread brakes. By acting directly on the steel running surfaces of wagon wheels, tread brakes can not only be noisy in themselves, but can lead to noise-producing irregularities in the tyre surface.

Tackling freight noise

Photograph of a train running along microphone array site in Switzerland.
The Silent Freight programme, co-ordinated by the European Rail Research Institute (ERRI) in the Netherlands, looked at a number of wheel designs with a view to cutting noise. A number of manufacturers worked with ERRI on different designs, with ideas including smaller wheels, perforations in the wheel, ring dampers, wheel shields and tuned absorbers. Different combinations of these features were tried. The use of ring dampers and wheel shields were seen to produce some of the best results, while wheel shape optimisation had disappointing results.
Quieter tracks

ERRI also co-ordinated the Silent Track project. Here, both retrofitting existing track and a new design of track optimised for noise reduction were studied:

  • The retrofitted section of track featured UIC60 rail - a heavy duty rail weighing 60 kg/m, which is approved by the International Union of Railways (UIC) for use on high-speed lines - with a stiff pad between the rail and the sleeper, and a tuned noise absorber.
  • The new section of track also used UIC60 rail, with a 100mm wide foot on the rail, rail head support, a tuned absorber and an optimised sleeper.

Combinations of the wheel and track designs were compared against wheels of a standard 920-mm diameter on ballasted track with UIC60 rail and soft pads. The overall objective was a 10dB(A) noise reduction, which implies more than a halving of the noise from a freight train.

The research results indicated that damping/shielding of wheels and damping of track are necessary to achieve any reasonable reduction in rolling noise. With the prototype designs developed during the project, for damped wheels on retrofit track the upper limit of noise reduction is approximately 7dB(A), while damped wheels on the new track design gave an upper limit of about 8dB(A).
In addition to these measures, a bogie shroud manufactured within international gauge constraints (168-mm gap to top of rail) was tested in combination with low height trackside barriers (50mm above top of rail); this gave a further noise reduction of 3dB(A).

The research showed that, when the best noise reduction measures are combined together, the 10dB(A) noise reduction target is achievable. The testing demonstrated that hard rail pads produce no additional benefit when used with damped track. The results of wheel shape optimisation were also disappointing. This gave railway engineers some indication about which ideas are not worth pursuing for noise-reduction purposes.

  New materials for brake blocks

Meanwhile, Eurosabot showed that there are no easy answers in the field of brake block materials. The aim of the Eurosabot project was to develop a brake material for tread braking of freight vehicles that would suppress the formation of wheel roughness, while still providing the required braking performance. Netherlands national rail operator NS was scientific co-ordinator on this project, with ERRI responsible for project management, and railways, manufacturers and research organisations from several countries helping in tests and contributing in other ways.

Cast iron is generally used for tread-brake blocks. This material has many properties that make it suitable for brake blocks, but it causes roughness on the wheel tread, and consequently a high level of rolling noise. The phenomena involved in the generation of roughness on the wheel surface were studied, mainly on the basis of dynamometer tests. Thermo-elastic instabilities due to local heating of the contact surface and material transfer between block and wheel, dynamic instabilities and periodic wear were felt to be factors responsible for the generation and growth of roughness.

A series of computer-based models was developed, that were able to reproduce qualitatively what had been observed in the tests. Together with these models, rig test results were used to select promising solutions from a large spread of ideas that were then tested in practice.

The researchers found that the results of the final tests were to some extent disappointing. The project did not succeed in identifying a 'low roughness' brake block material suitable for a simple retrofit exchange of cast iron blocks. However, it did succeed in improving significantly the understanding of the processes that are important in wheel roughness generation. In addition, advances were made in the modelling of thermo-elastic instability, combined dynamic-thermal-wear instability and brake squeal.

Tackling freight noise
Quieter tracks
New materials for brake blocks

Key EU-funded research

EU research on Land Transport and Marine Technologies is focussing on reducing rail noise of rail transport, particularly for freight.

Silent Freight
looking at ways to make freight vehicles quieter,

Silent Track
looking at ways to make the track quieter,

developing reliable methodology for designing tread brake blocks that would result in smoother wheel rolling surfaces.

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