Slab track is a modern form of rail or tram track. It is constructed without ballast (the coarse gravel laid to form a bed for tracks), a fact that explains its other name of ballastless track. There are several methods of slab track construction - click the link for full details.
Slab track has been used successfully throughout the world for high-speed lines, heavy rail (which generally travels across country carrying passengers and freight), light rail (mostly urban transport for passengers only) and tram systems.
Slab track is strong and very durable compared to other track structures. In terms of ‘whole life costs’ (the cost of laying the track and maintaining it) current research shows it to be a cost-effective alternative to ballasted track. Please click on the benefits of slab track question for full details.
What’s the alternative to slab track?
The traditional train track – known as ballasted track - is the alternative to slab track, where the rail is mounted onto a wooden or concrete sleeper. The sleeper sits on a bed of ballast (crushed rock), which distributes the loading to the subgrade (the prepared soil on which the track is constructed)
Top ballast is placed between the sleepers and on the shoulders to provide stability.
Traditional ballasted track has been used since the earliest days of the Victorian railways and has changed little in concept since that time.
The advantages of ballasted track are that it is relatively quick to lay and readily maintained by a fleet of specialist track maintenance equipment.
However, the nature of ballasted track means that the track can and will move with use so routine maintenance is always required to restore line and level, and clean or replace ballast regularly. This makes ‘whole life costs’ (the cost of laying the track and maintaining it) for ballasted track a significant draw back.
When was slab track introduced?
In the last 30 years there has been a considerable increase in the use of concrete slab technology because of advances in track technology, high speed trains and increased demand on the rail system, but the first recorded use dates back to 1899, when the Southern Railroad in American built a concrete slab under existing track in order to stabilise a section of rail on poor soil.
Records of slab track tests by Railway engineers date back to 1926.
What are the benefits of slab track?
Slab track is strong and very durable compared to other track structures. In terms of ‘whole life costs’ (the cost of laying the track and maintaining it) current research shows it to be a cost-effective alternative to ballasted track.
Very low maintenance requirement: Slab track systems require little routine maintenance. An inspection regime is, of course, necessary, but because the track is fixed in position there is no requirement for regular realignment of the rails. The very low maintenance requirement also means that track workers spend less time trackside, improving worker safety. There are examples of slab track installations where little or no maintenance (including rails and pads) has been carried out for over 25 years.
Shallow Construction depth: Many slab track systems require less construction depth than the equivalent ballasted system. This is an advantage in tunnels where headroom and gauge clearances are particularly important.
Reduced dead load: The shallow construction depth possible with slab track makes it lighter than traditional ballasted track, so on structures like bridges, there is reduced ‘dead load’, that is the weight the structure has to support constantly, which reduces costs.
Reduced structure gauge: Because slab track is fixed in position and will not move out of line or level, a reduced structure gauge (the minimum size of the tunnels, embankments and bridges) can be used with slab track, which reduces costs.
Higher speed operation: Concrete slab track offers a greater degree of trackbed stability than ballasted track. Therefore higher running speeds are achievable. Experience with use of traditional ballast at high speed (350 kph) has shown that fine particles can be sucked out of the track by the passing train. These particles are deposited on the rail surface and cause damage when run over by the wheels. In some areas this has required use of glued ballast to stabilise the track bed.
Engineered noise and vibration performance: The various configurations of slab track give it the design versatility to offer low noise and vibration whilst maintaining rail stability, depending on the needs of the surrounding area.
Long design life: An estimate of design life for traditional ballasted track is around 15 years, after which, the track requires renewal. A concrete track slab is typically constructed with a design life of at least 60 years. Increased reliability & availability: Slab track systems are more reliable than ballasted track, requiring little routine maintenance. Consequently fewer closures of the track are required for maintenance, increasing the availability of the track for running trains.
Low whole life cost: Although the capital cost of slab track systems is usually higher than the equivalent ballasted track, the long design life and minimal maintenance requirement for slab track systems means that overall their whole life cost is lower than that of traditional ballasted track
In the past, slab track systems were seen as expensive. While this is still true for the most sophisticated systems e.g. floating mass-sprung slab, for many systems the ongoing innovation and optimisation of slab track design is now reducing the capital cost to a level equivalent to ballasted track, without compromising performance.
A sustainable solution
In 2007, Britpave, together with NTEC (Nottingham Transportation Engineering Centre) carried out a comparative study into the sustainability of concrete slab track and traditional 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.
The study found that due to the long design life and low maintenance requirements of concrete slab track, it was the more sustainable option over a 60 year and 120 year lifecycle.
What does slab track mean to the passenger?
Assuming a high quality installation (as recommended by Britpave) the quality of the ride for the passenger on slab track will be smoother and more consistent when compared to traditional, ballasted track.
Because slab track requires less maintenance, there will also be less disruptions to the journey and more services available as the tracks will be available for trains to use more often. Services should also be more reliable as there should be no emergency speed restrictions caused be deteriorating tracks. A temporary speed restriction “TSR” is always applied following maintenance work on ballasted track.
Slab track is also more stable, so higher speeds could be achieved on the same alignment, reducing journey time. Fares could also be lower as less money would be needed for maintenance.
Finally, stations should be cleaner places as concrete slab track is easier to keep clean than ballasted track.
Are there any drawbacks?
There is a higher initial capital investment with slab track (though as previously mentioned whole life costs are low). Slab track takes longer to install and requires a high level of skill and workmanship.
Transitions between slab track and ballast require specialist design to ensure that the change in trackbed stiffness is managed. Britpave would recommend optimising the design to limit the number of different track systems and components used, and minimise the need for transitions.
Where settlements are expected, the slab track system needs to be designed to accommodate this. For example the Dutch high speed line HSL-Zuid was constructed across ground where a high degree of settlement was expected. Techniques to accommodate settlement include adjustable fasteners, use of shims and packing plates, and grouting.
How is slab track made?
There are five main categories of slab track, which can be a combination of concrete slab, sleepers and baseplates. Please use the menu on the left or click the link to look in detail at the five main types of slab track.
Construction techniques include slip-forming (the Britpave approach, see below), in situ concrete using fixed formwork, and use of pre-cast components. The rail and fastening system can be installed “top-down” (where the concrete slab is cast around the assembled baseplates and fixings) or “bottom-up” (where the slab is constructed first, and the rail and fastenings are fixed onto it) .
Slab track construction can be highly mechanised e.g. BBEST system or Dutch HSL-Zuid project. Other projects where short sections or a specialist track form is required use mainly hand laying techniques.
The shape of the slab can be varied, from a simple flat slab or one that incorporates precast concrete sleepers, plinths, derailment guards or embedded rail.
There are a number of slab track test sites and demonstration tracks across the world that have shown slab track to work, be cost effective and durable. Slab track systems have been proven in main line and high speed service for over 30 years.
What is Slipforming?
Britpave’s slipforming process allows a slab track to be created on site (in-situ). Ready-mixed concrete is poured into the front of a paver (effectively a large, slow moving mould), which places the concrete so that it emerges in the shape of the slab track from the back of the paver.
Is slab track suited to particular sites?
Britpave have identified several areas that are especially suited to slabtrack: new builds, tunnels, viaducts and major upgrades carried out on blockades.
Slab track is commonly used for paved track ways for light rail transit systems in urban streets where rights of way are shared with road traffic, in railway and transit tunnels where low track maintenance is crucial, and at level crossings to ensure a smooth crossing of the track by road vehicles.