Our towns and cities are increasingly becoming places where people want to live, as well as work, with a rising number next to rail routes. According to the United Nation’s ‘World Urbanization Prospects Highlights 2018’, 55% of the world’s population now lives in urban areas in 2018, compared to 1950 when the figure was only 30%. It is anticipated that by 2050, 68% of the global population will be living in towns or cities. Therefore, the challenge is to ensure sustainable urbanization as those 2.5 billion new citizens join cities by 2050.
New residential developments have sprung up in towns and cities across the world, attracting people to live closer to the transport network that moves them around. At the same time, governments are encouraging greater use of sustainable transport in and around cities and train, tram and metro travel is generally viewed as more efficient and environmentally friendly than motor vehicles as it does not create traffic jams and is not reliant on burning fossil fuels for energy and produces less CO2 emissions. All these factors have brought residents and the rail transport network closer together and, as a result of the increasingly urban population density, this has contributed to a rise in complaints about noise and vibration levels from the rail network. In addition, as many rail services now run 24 hours a day, so there is also an issue in some cities relating to noise at night.
The founding principle of rail transport is the low friction steel-steel solid contact between wheel and rail. This is the very reason for rail transport’s efficiency (low maintenance, high axle load…) but also its main burden, as this creates noise and vibration.
Direct noise in urban transport is primarily due to the rolling noise that is produced by train wheels and track as a result of their vibration. This noise travels through the air, from the railway line to people located nearby.
But this is not the only annoyance. The vibration produced by the solid contact between the steel of the wheels and the steel of the track goes into the ground and reaches into nearby buildings where it is “converted” in secondary noise when walls and floors vibrate and act as giant loudspeakers. This secondary noise is called structure borne-noise and is a problem for track neighbours – even with closed windows!
With brand new track and new rolling stock the noise is likely to be minimal, however as soon as there are even slight imperfections in the geometry and the surface of the wheel, or of the track, then vibrations arise. These soon become noticeable by local residents, leading to complaints.
As all operators know, drivers braking too hard, or leaves on the line that make the rails slippery, can cause wheel slips, leading to wheel flats that create the often heard ‘thump-thump-thump’ that can be heard by rail passengers and local residents as the damaged wheels impact on the hard rails beneath them.
Sharp curves in the track can cause major issues for those living nearby because of the squealing noise. This can be even more problematic in the morning, when each vehicle that exits the depot, which is generally full of sharp curves, generates high squealing noise as it runs over the curved track.
However, the most common, and some may say, the most irritating noise, is the ‘tac-tac’ sound from tracks that comes about as a result of a train running over a local defect. There are several potential causes. A driver accelerating too hard on a stretch of track can lead to wheel slip, which in turn creates a small indentation in the track. A poorly welded or ground rail, or rail fixed with fish-plates, will also lead to rail head defect.
Zones with a lot of braking or acceleration, and in some cases train curving behaviour on curves can lead to rail corrugation, where a defect becomes replicated at short intervals along the track, causing even more noise disturbance.
The other major cause of noise and vibration is that transmitted into the structure of a bridge or viaduct. This results in the structure producing noise that travels through the air to people below and besides it. If there are inaccuracies in the track geometry like the alignment, this can also result in vibrations.
Finally and despite noticeable improvement in their design, switches and crossings on the track are a source of significant noise and vibration, as they imply gaps and alignment changes to guide the wheels.
Whatever the many reason for the noise, a common characteristic is that once residents are sensitised to the vibration and noise levels affecting their properties, there needs to be a significant step change to mitigate the disturbance.
There are four ways, at track level, to limit noise and vibration annoyance of neighbours.
In general, the best way to mitigate the impact of noise and vibration is to integrate countermeasures into the design of a new or upgraded track. With noise and vibration, prevention is far easier than cure and mitigation needs to be carefully considered and built into all modern tramways, railways and metro systems.
Construction needs to be carried out carefully to avoid all those local defects generating noise and vibration (smooth track geometry, proper wheel design and rail interaction, proper welding and grinding of the rails, basic resilience in the track). Some level of resilience is generally incorporated into modern track, to attenuate the transfer of dynamic forces from the wheel / rail to the track support. This provides an initial track quality that aims to avoid generating vibrations.
Ensuring the quality and geometry of the track is essential to mitigating noise and vibration. In order to achieve this, the condition of the track needs to be constantly monitored and any maintenance and repairs must be carried out efficiently to reduce vibration and prevent it getting worse or causing secondary issues with the track. Pandrol’s Head Wash Repair (HWR) welding kit is a popular solution as it provides a quick and cost-effective solution to repair railhead defects and significantly decreases the maintenance cost of modern rail networks. The HWR process opens up new capabilities to railway maintenance by enabling removal of defects up to 1 inch, depending on the rail profile. For example, the repair of flash butt welds, which often suffer from squats.
Active wheel monitoring must be carried out to ensure train wheelsets are in a good state of repair and will not damage the track. This issue led to the development of WheelChex®, which allows management to gain a better understanding of the performance of train or metro wheels and allows proper design and maintenance plans to be put in place. WheelChex® is a single measurement device which integrates three measuring technologies and measures rail acceleration both vertically and laterally, as well as rail core temperature.
Beside track and wheels maintenance, introducing an elastic medium with specific spring characteristics, i.e. track resilience, will help to maintain track quality at a higher level over the life time of the track.
For example, in ballast track, Pandrol’s Under Sleeper Pads (USP) are tailor-made resilient systems designed to reduce track maintenance, increase the quality of the track and can also provide vibration mitigation by fixing elastic elements to the bottom surface of the sleepers.
Having a well-defined stiffness and/or continuous support of the rail will also reduce rail corrugation and the subsequent increase of vibration, as well as the need for maintenance grinding. This can be achieved thanks to continuously supported Pandrol’s QTrack® or high resilience base plates systems like Pandrol’s Vanguard and Pandrol Bonded DFF ADH.
If these first three measures are not sufficient, the vibration generated by the rail systems need to be further mitigated. The basic idea is to create a mass spring system with the track introducing an elastic medium with specific spring and damping characteristics that is decoupling the track. The result is that vibration energy remains in the track and is not transmitted to the neighbours.
To simulate the conditions and monitor the effectiveness of our solutions, Pandrol developed Track Elastic Model (TEM) software. This can also be utilised to simulate conditions at the transition between two different types of track and so smooth the design to avoid local degradation and therefore increased vibrations.
There are various levels of vibration reduction that can be achieved by different methods. These range from introducing soft fasteners, through to integrating very soft floating slab track, depending on specific design requirements and conditions.
Pandrol’s soft solutions include VIPA, Bonded DFF ADH, and DEE baseplates, and Booted Block and Under Sleeper Pads. These solutions help to reduce the impact and vibrations, in urban areas where requirements are low to medium. Soft Under Ballast Mats are another option to consider in case of ballast track.
The preferred solution of many metros is Pandrol VIPA DRS system, which is suitable for installation on non-ballasted tracks and areas where a reduction in vibration and secondary noise is required. This features a Pandrol e-Clip baseplate that is mounted on a studded natural rubber pad that provides the system resilience. Within limits the configuration can be tuned to meet requirements on axle loads and stiffness. The VIPA DRS has been installed in major cities such as Bangkok, Chennai, Delhi, Dubai, Hong Kong, Istanbul, Kolkata, Kuala Lumpur, Sao Paulo, Seoul and Singapore.
For higher attenuation requirements, the Pandrol VANGUARD and Floating Slab Mat (FSM) solutions are recommended.
Pandrol Vanguard is a rail fastening system with very low vertical dynamic stiffness that leads to high levels of vibration isolation. It is suitable for use on concrete or timber sleepers, slab track on bridges, tunnels and viaducts. Its advantages are that it is a very low-profile system, that can easily be retrofitted, with various footprint designs, and it offers high levels of lateral and vertical adjustment. A number of cities, including Barcelona, Madrid, Milan, London, Stockholm, Sydney, Sao Paulo, Philadelphia and Boston have implemented retrofit installations on their railways using this system. In several cases it has been shown that reductions in noise levels in buildings of the order of 10 to 12 dBA have been achieved. This has been enough to reduce the noise to a level where it is barely perceptible and to eliminate complaints completely.
Floating Slab Mats are installed bellow the slab track to provide excellent vibration mitigation as this create a very efficient mass spring system working perfectly both in day conditions (fully loaded vehicles) or night conditions (empty vehicles with very demanding noise limits).
As an example of this, the Brussels Tramway STIB-MIVB chose to install Pandrol’s Floating Slab Mat (FSM) to mitigate noise and vibration. Over 150,000 m2 of FSM has been installed over busy urban sectors with limited intervention time. The requirements were extremely high as local residents regularly complained about tram noise and vibration in busy narrow streets. Since the FSM solution was implemented, the level of complaints from residents about noise and vibration has been minimal. A number of others cities have implemented these solutions across the world including Chennai, Sydney, Portland, Los Angeles, Toronto, Athens, Lisbon, Madrid, Roma, Milan, Florence, Bergen, Budapest, Szeged, Sofia, Alger, Casablanca, Rio, and Santiago.
In the most demanding areas, an even more efficient mass spring system is the Floating Slab Pad, where the resilient mats are substituted for softer discrete pads. This implies the use of precast slab track making the solution more expensive, but provides a premium level of vibration mitigation and a system that is easy to install and renew. This system is ideal in any highly demanding tunnel project. The city of Barcelona is using this system since the late 1990s to the full satisfaction of its citizens.
Since rail transport is an important part of a low carbon transport system and helps to reduce road traffic congestion in a world where now over 4 billion people live in cities, minimising the noise and vibration it causes in urban areas is essential.
Good design and maintenance are the key to this and effective noise and vibration solutions are being integrated within the new and existing rail networks within many towns and cities, enabling residents to live peacefully alongside.
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