For the past 150 years, railway track has been designed to use steel rails attached to transverse sleepers which are, in turn, embedded in stone ballast. This ballasted track design has been used, relatively unchanged, to build the world’s railways.
Railways Africa spoke to Craig Tengstrom, Contracts Director for Tubular Track (Pty) Ltd, about Tubular Modular Track (TMT).
In the 1980s in South Africa, Peter Küsel, a civil engineer with many years of experience in the rail industry, considered that there must be an alternative way to build railway track without ballast and sleepers. He set about developing an alternative railway track system. This saw Tubular Modular Track (TMT) evolve over the years to what it is today. Tubular Track (Pty) Ltd has its headquarters in Pretoria, South Africa.
The design that Peter developed first of all dispensed with the use of ballast and sleepers. Instead, the track is supported on twin, pre-cast reinforced concrete beams which are placed under, and in line with, the rails. The rails are attached to the beams using purpose-designed components called stirrups and gauge bars, the gauge bars also maintaining the track gauge. Standard proprietary fasteners are used to fix the rails to the components.
Ballasted track’s resilience is in the ballast below the sleepers. Unfortunately, fouling of the ballast over time negates resilience, and deterioration of the track system sets in, ultimately leading to exacerbated stresses in the system and possible component failures. TMT, on the other hand, incorporates a continuous resilient pad placed between the rails and the beams. This interface area is impervious to any foreign matter and therefore resilience in the system is maintained even in adverse conditions.
This is very evident in the installation in the desert areas of Saudi Arabia where TMT was inspected 12 months after installation and the integrity of the pad was not compromised due to sand ingress.
Resilient pads were recently replaced on a 1:12 turnout that had handled in excess of 360 MGT. While the pad showed signs of heavy use, it had by no means reached a point of failure. “What is interesting”, explains Tengstrom, “is that if one applies this tonnage benchmark to passenger and other freight lines, the resilient pad should last for decades”.
Extensive testing of Tubular Modular Track by independent experts has shown that rail stresses are reduced by up to 75% compared to ballasted track systems. This is possible due to the rails being continuously supported by the concrete beams.
Because the rail stresses in TMT are so much lower, it is possible, when installing TMT, to use lighter rail sections, thus offering clients significant savings in rail procurement costs. In Tubular Track’s recent successful Namibian project, this benefit was highlighted when second hand 30kg/m rails were used for an 18.5 ton axle load application, providing the client with huge savings.
TMT’s axle loading capacity is not a limitation. A large iron ore mining company in Australia commissioned a design for a 36 ton axle load application. In the technical study, TMT was deemed to be completely acceptable for carrying this exceptionally high axle load.
“Because the stresses in all components of TMT are greatly reduced, there are significant savings to be had,” says Tengstrom. Importantly, geometric stability of the track system is greatly enhanced, particularly in turnouts.
“Because of TMT’s fixed geometry, I can say with confidence that TMT turnouts perform exceptionally well when compared to ballasted turnouts,” he explains. “This is evident in the UIC60 1:12 Tubular Modular Turnouts that were installed in the Ermelo Yard (26 ton axle loads) which to date have carried in excess of 360 MGT while requiring minimal maintenance”. (Rail grinding was only done once in this period).
Significant savings are also to be realised in the earthworks component where the formation width is narrower, offering up to 40% savings in construction costs. Formation design essentially follows the guidelines of Transnet’s S410 specification with the exception that the formation width is reduced to 3m for single track at a gauge of 1,067mm.
One might ask how the smooth transition of TMT to ballasted track is achieved.
Tengstrom explains that R&D plays an important role in the company to continually improve the product. One of the projects was to find a truly workable solution to the challenge of transition points where the stiffness of the systems differ. “A solution has been developed and with minor adjustments to suit each case, transitions are dealt with comfortably.”
Tubular Track uses the Transnet Freight Rail (TFR) Track Test Centre for ongoing testing of the system, and their association with this centre of expertise goes back to the early 1990s. All TMT installations are designed in conjunction with professional engineers from the geotechnical and structural disciplines to meet the client’s track usage requirements.
Tubular Track has launched a new R&D test site at the Burnett Street Metrorail deviation in Pretoria, in conjunction with the University of Pretoria and Transnet’s Freight Rail Track Test Centre. Three monitoring test points have been set up with the full support of the Passenger Rail Agency of South Africa (Prasa).
These monitoring test sites will measure deflections in the formation layers and in-situ material up to 2.5m deep using Multi Depth Deflectometers (MDDs). The MDDs will also be used to measure the permanent deformation of the formation layers which will be used to calculate the formation life. An array of other measurements will also be monitored including stresses in components, lateral forces on the rail, etc. This will be a long term test site and valuable information will be gathered, analysed and ultimately used to improve the performance of TMT.
Commuter safety is, of course, a crucial aspect for any rail authority. In a joint project, Transnet Freight Rail, Prasa and the Rail Safety Regulator have published a new specification for Metro platforms on Transnet approved ballastless track. This specification essentially determines a new height between top of rail and platform resulting in the floor of the coaches being at the same level as the platform. TMT fulfils all the requirements to meet this new specification, and a number of platforms have already been installed to this specification. To achieve the tolerances required by this new specification, track geometry has to be permanent to prevent coaches coming into contact with the platform, and therefore excludes ballasted track where loss of geometry occurs.
Following the installation of a section of TMT in Saudi Arabia in 2008, there is growing interest in the Middle East, as the benefits of TMT are very tangible in these desert environments. High installation rates are the order of the day and need to be maintained. Tubular Track is confident that installation rates in the order of 750m/day can be achieved as installation can be done at multiple points along the line. TMT does not rely on the completed line (single-point entry) to source and feed material to the site. Therefore incomplete civil structures such as culverts and bridges will not interfere with installation programmes as these areas can be leap-frogged and completed at a later stage, once the structures are complete.
Maintenance of railway lines is a very costly component of the life-cycle costs of any track system. Extensive capital expenditure in plant and resources is required to maintain ballasted track over time with the bulk of the costs being ballast cleaning, replacement and tamping. TMT, being ballastless, requires minimal maintenance below the rails, and maintenance is not plant-intensive. Expected savings on maintenance can be up to 60% compared with ballasted track.
“Ultimately, TMT offers many areas of savings, in particular, formation construction, rails and maintenance”.
“Had TMT been in existence 150 years ago, it may well have been the first choice of the world’s railway engineers,” Tengstrom concludes.