Our towns and cities are increasingly becoming places where people want to live, work and spend their leisure time, with a rising number choosing to live next to rail routes for the convenience this offers.
According to the United Nation’s World Urbanization Prospects Highlights 2018, 55% of the world’s population already lives in urban areas, compared to around 30% in 1950. It is also anticipated that 68% of the global population will be living in towns or cities by 2050. The challenge therefore is to ensure sustainable urbanisation – as those 2.5 billion new citizens relocate to cities in the coming decades.
As new residential developments have sprung up in towns and cities across the world, and suburban growth continues, the attraction of living closer to the transportation networks that move them around these densely-populated areas is obvious.
To cater for these needs, governments and city authorities are encouraging the greater adoption of sustainable transport modes in and around these expanding urban settlements. Rail-based solutions such as train, tram and metro services are generally viewed as more efficient and environmentally-friendly than rubber-tyred motor vehicles as they do not contribute to already problematic congestion issues and are not reliant on burning fossil fuels for energy. They also produce fewer CO2 and other harmful emissions.
All these factors have brought residents and rail networks closer together. As a result of this growing urban population density, we have seen a rise in complaints about noise and vibration levels from these networks. Additionally, as increasing numbers of rail services now run almost 24 hours a day, there are issues related to the noise generated at night when the limits of what is deemed acceptable are dramatically reduced.
What causes noise and vibration?
The founding principle of rail transport is the low friction steel-on-steel contact between wheel and rail. This is the very reason for the mode’s efficiency (low maintenance, high axle load etc), but also its fundamental burden as it is this interaction that creates noise and vibration.
Direct noise related to urban transport is primarily due to the rolling noise produced by train wheels and track as a result of their vibration. This noise travels through the air, from the source of the railway line to both residents and businesses 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 also passes into the ground and reaches into nearby buildings where it is ‘converted’ into secondary noise when walls and floors vibrate and effectively act as giant loudspeakers to magnify the effects (see above). This structure-borne noise can be a significant problem for those located close to rail lines… even with their windows closed.
With brand new infrastructure 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, vibrations arise. These soon become noticeable by local residents, leading to a rise in complaints.
And as any rail operator will know, harsh braking or leaves or other contaminants on the track that make the rails slippery can cause wheel slips, leading to wheel flats that create the often-heard ‘thump-thump-thump’ heard by both passengers and local residents as the damaged wheel areas strike the hard rails beneath them. Sharp curves in the track alignment can also cause major issues for those living nearby because of the common issue of ‘curve squeal’ which is experienced if the track is not properly designed, installed or maintained. This effect can be even more problematic in the morning, when each vehicle that exits the depot – which is generally full of sharp curves – generates a high-pitched squealing noise as it runs over the curved track alignment.
However, the most common, and some say most irritating, noise, is the ‘tac-tac’ sound that results from a vehicle running over a local defect. There are several potential causes of this effect. A driver accelerating too hard on a stretch of track can also lead to wheel slip, which in turn creates a small indentation in the track. A poorly-welded or ground rail, or rail fixed with fishplates, will also eventually lead to rail head defects. Sections that feature a lot of heavy braking or acceleration, and in some cases a vehicle’s behaviour on curves, can lead to rail corrugation. This is where a defect becomes replicated at short intervals along the track, causing an even greater disturbance.
The other major cause of nuisance noise and vibration is that transmitted into the structure of a bridge or viaduct; this results in the structure magnifying the effects of noise travelling through the air to people below and beside it. Inaccuracies in the track geometry can also result in similar effects.
Finally, and despite noticeable improvements in their design, switches and crossings are a source of significant noise and vibration, as they imply gaps and alignment changes to guide the wheels.
Whatever the reason for the noise, a frequent characteristic is that once residents are sensitised to the vibration and noise levels affecting their properties, there needs to be a step-change to mitigate the disturbance.
How do we control it?
There are four ways, at track level, to limit noise and vibration annoyance to those in close proximity to an urban rail line:
• Deliver a new state-of-the-art rail system
• Monitor and maintain the existing system
• Reduce track degradation by adding track resilience
• Mitigate noise and vibration.
The best way to lessen the impact is to integrate countermeasures into the design
of a new or upgraded track. Prevention is always far easier than cure and mitigation needs to be carefully considered and built into all modern tramways, railways and metro systems from the earliest design phases wherever possible.
Integrate noise and vibration solutions into track design
Track construction needs to be carried out carefully to avoid all those local defects that can generate unwanted effects: smooth geometry, proper consideration of the wheel design and rail interaction, high-quality welding and grinding of the rails and basic resilience in the track.
Some level of resilience is generally incorporated into modern track designs, 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 – and the noise resulting from them.
Monitoring track and wheel quality
While noise and vibration can never be eliminated entirely, ensuring the quality
and geometry of the track installation is essential to reducing their effects. In order to achieve this, the condition of the track needs to be constantly monitored with any maintenance and repairs carried out promptly and efficiently. This preventative maintenance will reduce vibration issues and prevent them worsening or causing secondary issues with the track.
Pandrol’s Head Wash Repair (HWR) kit is a popular solution as it provides a quick
and cost-effective solution to repairing railhead defects, significantly decreasing
the maintenance cost of modern rail networks. Reducing the amount of welds by using specially-developed moulds, this opens up new capabilities by enabling the removal of defects up to 25mm (one inch), depending on the rail profile – for example, the repair of flash butt welds, which often suffer from squats.
Active wheel monitoring must also be carried out to ensure that vehicle wheelsets are in a good state of repair and will not damage the track. This issue led to the development of solutions such as WheelChex®, a measurement device mounted at the lineside that integrates three measuring technologies to assess rail acceleration vertically and laterally, as well as rail core temperature. By measuring the impact upon the railhead of each wheel of a passing rail vehicle (for example, if it has a flat), such technologies allow operators and engineers to gain a better understanding of the performance of a vehicle’s wheels and for the preparation of appropriate maintenance regimes.
Track quality control by resilience
Besides infrastructure and vehicle maintenance, introducing an elastic medium with specific spring characteristics – i.e. track resilience – will help maintain track quality at a higher level for longer periods.
For example, for ballasted track, Under Sleeper Pads (USP) are tailor-made resilient systems that are designed to reduce maintenance interventions. Placed between the sleeper and the ballast, USPs improve the ride of the vehicles and provide a reduction in vibration 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 consequent increase of vibration, as well as the need for maintenance grinding. This can be achieved thanks to systems such as QTrack® or low resilience baseplates. One of the advantages of such systems is that their stiffness levels can be carefully adjusted to achieve the specific attenuation levels required.
Track isolation principles
If these first three measures are not sufficient, then the vibration generated by the rail systems needs to be further mitigated. The idea is to create a mass spring system with the track introducing an elastic medium with specific spring and damping characteristics to ‘decouple’ the track with the result that vibration energy remains in the track and is not transmitted to neighbouring structures.
To simulate the conditions and monitor the effectiveness of our solutions, we developed Track Elastic Model (TEM) software. This can also be used to simulate conditions at the transition between different types of track and thereby smooth the design to avoid local degradation.
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 the design requirements and individual conditions that are dependent on site specifics.
Soft solutions include various baseplates and under-sleeper pads that help reduce the vibrationary impacts of passing vehicles in urban areas where requirements are low to medium. Naturally, the more intensive the services, especially prevalent in cities, the greater the potential issue.
The preferred solution of many metros is our 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 mounted onto a studded natural rubber pad. Within specified limits the configuration can be tuned to meet requirements on axle loads and stiffness and this system also exhibits a high level of electrical insulation. Components can also easily be replaced in situ.
Rail fastening systems such as Vanguard offer very low vertical dynamic stiffness that in turn means high levels of vibration isolation. They are suitable for use on concrete or timber sleepers, slab track on bridges and viaducts and in tunnels, and offer a very low-profile that can easily be retrofitted. This system has been installed in cities such as Barcelona, Madrid, Milan, London, Stockholm, Sydney, Sao Paulo and Philadelphia; under optimal conditions it has been shown that reductions in noise levels of the order of 10-12dBA have been achieved. This has been enough to reduce the noise to a level where it is barely perceptible, thereby eliminating complaints from nearby residents entirely.
For higher attenuation requirements, Floating Slab Mat (FSM) solutions can be used. These are installed bellow the track base and provide excellent vibration reduction, creating a very efficient mass spring system working perfectly both during the day (fully-loaded vehicles) or at night (empty vehicles but with very demanding noise limits).
As an example of this approach, STIB-MIVB selected an FSM installation to mitigate
these issues on Brussels’ tramway with over 150 000m2 installed over busy urban sectors with limited intervention time. The noise attenuation requirements were extremely high in these locations as local residents regularly complained about tram noise in busy narrow streets. However, since the installation the level of complaints from residents has been minimal.
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 this provides a premium level of vibration management through a system that is easy to install and renew. This is ideal, for example, in any highly-demanding tunnel project. The Spanish city of Barcelona has been using this system on its metro since the late 1990s.
Since rail transport is an important part of any low carbon transport system, and helps reduce road traffic congestion in a world where over four billion people now live in cities, minimising the noise and vibration it causes in urban areas is essential.
Good design and maintenance are key,and effective solutions are being integrated within the new and existing rail networks within many towns and cities, enabling residents to live peacefully alongside.
Article originally appeared in TAUT 974 (Feb 2019).