A sustainable track solution

The new government’s high speed rail vision needs 21st century track system, says David Jones, director, Britpave

The good news for the future development of a UK high speed rail network is that the new Conservative/Liberal Democrat coalition government has pledged to deliver such a network as part of its plans to achieve a low carbon economy. The bad news is that unless the network is built using 21st technology the patch and mend problems with our current network will continue.
    
The new government announced its commitment to the establishment of a high speed rail network within a programme of eco-friendly measures. The network would link London with Glasgow and Edinburgh and provide significantly reduced journey times. However, the wrong type of track could scupper these plans.
    
“Britain needs a 21st century rail network and for that we need a 21st century track system”, said David Jones, director of Britpave, the transport infrastructure group. “The use of the ballast track system, which is a 19th century approach, could undermine the speed, efficiency and safety of high speed trains.”

Different solution
The French opted for traditional ballast when the high-speed TGV network was developed. They have managed to make it work – but at a cost. On some lines the rails have to be swept clean and the ballast replaced and repacked every night. The French are now reported to be examining replacing ballast with concrete slab track.
    
Jones believes that concrete slab track, as used by the highly successful Japanese rail network and increasingly throughout mainland Europe, is the way forward. “Concrete slab track maximises operating efficiency by eliminating unplanned maintenance, provides high levels of safety and comfort and impressive long-term performance,” he said.
    
Traditional ballast track has been used since the earliest days of the Victorian railways and has changed little in concept since then. It has the advantage of being relatively quick to lay and can be readily maintained by a fleet of specialist plant. However, the nature of ballast track means that the track can and will move under load which results in the need for on-going maintenance to restore the line and level and for the ballast to be cleaned or replaced. Also, experience of the using ballast at high speed (350kph) has found that fine particles can be sucked out of  the track by the passing train. These particles are then deposited on the rail surface and cause damage to the train’s wheels. With concrete track slab systems, the ballast is replaced by a rigid concrete track slab, which transfers the load and provides track stability. Resilience is introduced into the track system by means of elastomeric components. These may be pads, bearings or springs depending on the type of slab system.

Types of systems
There are broadly five types of slack track system. These include embedded rail, booted sleepers, direct fixing and resilient baseplates, cast-in sleepers and floating slab. All reduce the need for maintenance as because the track is fixed into position there is no need for regular rail realignment. This means that fewer possessions of the track are required for maintenance. In addition, many track systems require less construction depth than the equivalent ballasted system. Embedded rail systems and resilient baseplate track types require the least depth. This is an advantage in tunnels where headroom and gauge clearances are particularly important issues. The fixing of the track so that it will not move out of line has a further advantage, a reduced structure gauge can be used which means that not only can tunnel bore dimensions be reduced but running speeds can be increased.
    
“The range of construction benefits using concrete slab track are matched by the performance benefits of reliability, speed and smoothness of the ride,” said Jones. Slab track can be designed to meet stringent noise and vibration criteria. A slab track system, such as booted sleepers or floating slab, can be designed to meet the requirements of locations sensitive to ground-borne vibration. For each slab track system, the resilient components can be optimised to ensure the best balance between acoustic performance and rail stability.
    
Concrete slabtrack has been used in the UK for tunnels and for short stretches of the Heathrow Express, Stansted airport links and at the Eurotunnel terminal. Initial cost has always been given as the reason why slabtrack is not more widely used in the UK. “This is short-sighted and a false economy and, thanks to ongoing slabtrack development, incorrect,” said Jones. “The maintenance costs of slabtrack are dramatically less than ballasted systems and the long-term performance is significantly superior which means that over the whole life of the slabtrack its cost is considerably less than that for ballast. Furthermore, the ongoing innovation and optimisation of slabtrack means that capital cost is often less than ballasted track.”
    
The design life for traditional ballasted track is some 15 years to compared to the 60 years for concrete slabtrack. The long design life of concrete slabtrack means that it is the best long-term sustainable option. A study carried out by Britpave together with NTEC compared the sustainability of concrete slabtrack with that of ballasted track. The study looked at an environmental life-cycle analysis through the whole life of the track including source of materials, manufacturing, construction, maintenance, decommissioning and recycling. It found that due to the long design life and low maintenance requirements of concrete slab track, it proved to be the more sustainable option over a 60 year and 120 year lifecycle.
    
“Between 2008 and 2009 1.27 billion rail journeys were made and rail passenger numbers are forecast to double over the next 30 years,” said Jones. “Our outdated rail network is simply not up to job and to continue to use a 19th century-based track system will simply exacerbate the problem. If the new government is serious about delivering a 21st century high speed rail network then it must invest in a 21st century track system.”