Although ballasted track remains the conventional method of constructing railway lines, the system is susceptible to ballast contamination in desert and wet areas as well as high-wear in heavy use applications. This can lead to high maintenance costs and, sometimes, even track failure.
Since the 1950s, ballast-less track systems have evolved for specialized applications such as high-speed rail and tunnels, but at a significant initial cost premium. What is needed is an affordable solution for areas with poor ground conditions for which conventional ballasted track is not suited.
Ballasted or ballast-less?
Conventional ballasted track consists of cross-lying timber, steel or concrete sleepers on a ballast bed of crushed quarry stone which in turn lies on a wide earthworks formation. Essentially, it is a bridging system whereby sleepers resident on resilient ballast are bridged by rails that are strong (heavy) enough to accommodate the bending stress required by the train’s axle load specification.
Ballasted track is expensive to maintain given that it contaminates easily, requires access to quarries and ballast supply trains and has to be cleaned and renewed using specialised machinery. All of these can present significant challenges to those responsible for railway construction and maintenance in developing countries and in areas of difficult terrain.
Different ballast-less track systems are available and being used around the world, mostly in the category of track slab systems. These include systems with discreet rail support, such as a hand-laid reinforced concrete slab with baseplates; RHEDA2000 – where standard sleepers are cast into a continuous slab; Sonneville Low Vibration Track (LVT) and the Japanese reinforced concrete roadbed system (RCRS) that uses precast concrete slabs.
Ballast-less track systems with continuous rail support include Paved Concrete Track – PACT and Embedded Rail Structure (ERS) where the rails are embedded in an elastic boot, cast into a continuous concrete slab.
The technological debate between continuous vs. bridged support of rails has some of its origins in Brunel’s “Baulk Road” system installed in the 1800s on the Great Western Railway in the UK. In these systems, each rail is supported along its entire length by a baulk or longitudinal sleeper. Gauge is maintained by the use of tie-rods between the two baulks.
The T-Track system, privately developed in South Africa since 1989, is a ballast-less track system that competes on an equal footing with conventional ballasted track on initial and major upgrade costs, whilst maintaining the substantial cost advantage of ballast-less track. It is an integrated track system comprising both the track substructure, characterised by narrower layer works (compared to conventional ballasted track) and the track superstructure, characterised by a modular articulated beam-track arrangement.
The basis of the T-Track system is the track module. This consists of a pair of reinforced concrete beams which are wet-cast into geotextile bags and separated by gauge bars. Stirrups take conventional rail fasteners (Pandrol, Vossloh, Unit Rail) and continuous pads along the length of both beams provide a resilient base for the rail itself.
The modules sit directly on a prepared formation that is narrower than that required for ballasted track and are grouted into place. The design and stiffness of the modules and the formation is carefully calculated using finite element analysis to obtain the optimum performance and support for the track modules.
There are three basic designs of module. Tubular Modular Track (TMT) is used on straight and curved sections. Modules are precast using the same mould.
Modular Tubular Turnouts (MTT) are precast in sections using a double-sided mould to accommodate left and right-hand turns. Modular Tubular Level Crossings (MTLx) – these are an adaptation of TMT using precast inserts which fill the void between the beams to establish the level crossing.
All three types are available for standard gauge (1,435mm) and narrow gauge (1,067mm and 1,000mm) although other designs are perfectly feasible. MTTS are designed for UIC and voestapline VAE designs for 1:9, 1:12: 1:20 secant and tangential turnout sets, and will soon be availiable for the range of Vossloh turnouts.
The T-Track structure is designed using a conservative 2D model to an ultimate limit state (including fatigue). This is verified against a 3D model concurrently during formation design. The serviceability limit state is set at soil deformation. 3D tie beam design and analysis is verified by hand calculation.
Design considerations include the required axle load and speed, rail size and the underlying soil stiffness.
The rail-track with the formation is modelled, with three dimensional (3D) finite element models (FEM), to determine the stresses and deflections (behaviour) of the track structure. The FEMs are solved with a transient dynamic solver, with the linear elastic material properties with damping coefficients for the system with moving load applications.
The FEM has been calibrated extensively over a number of years for heavy haul operations by the Transnet Freight Rail Track Test Centre in South Africa, as well as the University of Pretoria using measurements from instruments such as pressure plates/ cells at the interface between formation layers and multi-depth deflectrometers (MDDs), typically at three stations, with three holes per station and six MDDs per hole.
In use today
T-Track exhibits a low initial cost and overall lowest total cost of ownership when compared to other track systems. Project cost compares favourably with ballasted track, but has lower maintenance cost, exhibiting a doubling in rail and turnout life and an improvement in weld life. The impact of Tubular Modular Track on project cost is significantly less than slab-track, as it requires a narrower formation as ballasted track. Tubular Modular Track project costs decrease with increasing use over the length of track, as less transitions are required and economies of scale in manufacturing and installation logistics increase.
The ballast-less T-Track system represents a major cost breakthrough for use as a generally affordable rail track solution. It has developed a strong multi-disciplinary value proposition to displace conventional ballasted track and slab track systems in many cases.
And it is not new, unproven technology. 600km are in use in the mining industry while the first surface track was laid for freight traffic in 1990. Since then there have been many successful applications in South Africa, USA and Canada with some installations conveying in excess of 80 million gross tonnes per annum with axle loads of up to 32 tonnes.
Turnouts have been in operation at Ermelo Coal Yard since 2000 without any significant maintenance and have now carried more than 950 million gross tonnes.
Ideal in sandy environments, the first T-Track was installed in Saudi Arabia in 2008. Not only does it remain perfectly resilient, it has maintained its original geometry within 2mm tollerance. Today, having endured some 125 billion gross tonne kilometres in extreme conditions over many years, this technology is now fully proven and certified for operations.
With its lowest cost of ownership and scalability into large projects this technology could well revolutionise the way rail track, especially on freight lines and in difficult terrain, is constructed in future.