In densely populated conurbations, noise mitigation is top of the agenda for traffic planners. In accordance with the goal of transferring more traffic from roads to rail, network operators – especially local public transport—are making huge efforts to provide protection from noise. Vossloh has developed innovative technologies for “quiet tracks” that appreciably reduce structure-borne and secondary airborne noise. This is achieved, for instance, by employing special inserts like cellentic in highly developed fastening systems, rail web damping systems, and rail grinding to improve acoustics. The interplay of coordinated measures and technologies reduces the need for expensive noise barriers and additionally enhances travel comfort.
Modern rail fastening systems offer the necessary elasticity to ensure that forces due to eccentricities in the wheels or irregularities in the rails do not lead to excessive wear or damage, which in turn causes vibrations and structure-borne noise. A material called cellentic from Vossloh Fastening Systems is a key component in the reduction of structure-borne noise. This material is applied in the form of rail or base-plate pads. Thanks to its closed-pored structure, this highly elastic microcellular EPDM (ethylene propylene diene monomer) efficiently and durably prevents vibrations from getting through to the substructure. The chemical composition of Vossloh’s cellentic has a saturated polymer main chain, which provides a high level of resistance to a large number of chemical attacks. It also accounts for the material’s good temperature, weathering and aging behavior. Microcellular rail pads and base-plate pads made of cellentic retain their original shape even when subject to repeated loading. (Fig. 1)
In the Budapest metro project described below it was, in particular, the low-frequency response (between 1 and 30 Hz) under dynamic loads that played a decisive role.
Use of cellentic in Budapest
At present, the metro network in Budapest has a total length of 33 kilometers and 42 stations. Every day, it is used by some 770,000 passengers. Construction work on a new line, the M4, has been underway since 2004. This new line is intended to relieve road transport in the Hungarian capital and its first section is scheduled to enter service in 2014.
In building the new line, tough targets were set for keeping noise under control, because in some places the walls of the metro tunnel are a mere two meters away from the foundations of neighboring buildings. The company that operates the metro, Budapesti Közlekedési Zártkörűen Működő Részvénytársaság (BKV Zrt.), therefore defined nine critical sections of the line calling for special treatment.
It was decided to deal with this situation by laying highly elastic trackage with closely matching components that work together seamlessly. For this purpose, Vossloh Fastening Systems developed three different versions of the rail fastening system 336. (Fig. 2) For all of them, the principal consideration was the physical law whereby “the more elastic the rail fastener, the lower the structure-borne noise”.
The following were the applicable design criteria:
• unballasted tracks with elastic rail supports,
• a clearance of 750 mm between rail supports and
• compatibility with the metro trains’ maximum axle load of 12.5 t.
The enhanced systems are based on rail fastening systems for ribbed plates, which have been in use for some 30 years on conventional, metro and urban light rail lines in more than forty countries all over the world. On account of its flexibility, this system can be readily adapted to local circumstances. System 336 features the Skl 24 tension clamps. The steel and cast-iron ribbed plate rests on a cellentic rail pad with a stiffness of at least 8 kN/mm. The unit is prestressed to precisely the required amount by tightening a spring. The rail fastener was adapted for the Budapest metro in accordance with the customer’s specifications. The System 336 fasteners used for the M4 line, including six of the nine critical sections, incorporate a 15-mm thick elastic pad with 10 kN/mm stiffness. Vossloh achieved this stiffness by making appropriate modifications to the air bubbles enclosed in the cellentic.
Vossloh Fastening Systems delivered a total of some 38,000 rail fasteners of this type for the new line up to the beginning of May 2012. A total of 15 switches and crossings for the Budapest metro were also equipped with Vossloh’s System 336. The rail supports are fastened to the track using a top-down method. The rails and their fasteners are first of all aligned and only then is the concrete cast around and/or under them. (Fig. 3)
For the three particularly sensitive sections (where neighboring buildings are so close that they would be severely affected by structure-borne noise), the unballasted trackage has been constructed as a mass-and-spring system tuned to a frequency of 16 Hz. The base-plate pads here are even stiffer at 17 kN/mm.
Acoustic tests, independent technical tests and approval procedures were performed on Vossloh’s System 336 by local institutes in accordance with the EN 13481-5 for fastening systems for slab tracks (including on metros).
Further “quiet system projects” for light-rail slab tracks
After successful completion of the Istanbul Metro project, Vossloh Fastening Systems won another W-Tram rail fastener project in Turkey: for the Bursa Metro. For this purpose, W-Tram rail fasteners have been supplied since mid 2012.
The W-Tram is a high-elasticity rail fastening system for local transport; it is anchored to the subgrade with screws/dowels and can be used both on turnouts and covered tracks. It will accept a maximum axle load of 13 t. The system consists basically of three components (Fig. 4). The Skl 21 tension clamp has two free spring arms which have a long elastic deflection and securely hold the rail on the concrete body in a frictional connection. The tension clamp has fatigue strength of 2.5 mm and a high toe load which also ensures the high creep resistance of the rail in the entire system. Moreover, the defined fatigue strength also makes sure that the clamp will withstand countless loadings across a certain peak-to-valley value without suffering any damage. The angled guide plates hold the rail laterally in position. And together with the middle bend of the tension clamp they prevent the rail from tilting or lifting. The high-elasticity rail pad discharges the dynamic forces vertically to the sleeper. It has a stiffness of 30 kN/mm and allows ample vertical subsidence. It consists of the patented microcellular EPDM elastomer marketed by Vossloh as cellentic. Among other things the material is remarkable for its outstanding absorption of structure-borne noise, its excellent UV and ozone resistance and its very good resistance to aging and weather since the closed cellular texture will absorb very little moisture. Hence, the life-cycle costs are correspondingly low.
The system allows width and height adjustments. Height-adjustment washers are simply inserted from the side. For re-gauging by +/-10 mm, fully insulated angled guide plates of various widths are used.
For higher loads of up to 22.5 t, Vossloh has developed the W-Tram into the system DFF 21. It serves as a simple fastener for conventional slab tracks and is primarily installed top-down. It is presently being used for instance on the Ankara Metro and on the Piraeus project at this, the third biggest Mediterranean port. It has a stiffness of 60 kN/mm and is installed together with the Skl 21 tension clamp. The DFF 21 is a cost-efficient direct fastener for slab tracks; it is simple to install and allows vertical forces to be discharged along an underlying elastic base plate. Good electrical resistance is among its hallmarks.
Concluding summary
The numerous projects from Vossloh show that slab tracks are no longer only found on high-speed lines but nowadays also frequently used on light rail systems. Especially in tunnels, slab trackage is employed for its greater rail stability and lower maintenance costs. Besides being used on subway or metro lines, slab trackage is also employed on light rail lines and numerous tramways. Here, too, the rail fastening systems must feature a high degree of elasticity so that the forces discharged by wheel runout or rail irregularities do not lead to excessive wear or damage. Rail pads and base-plate pads made of cellentic from Vossloh Fastening Systems are helping to minimize vibrations and abate structure-borne noise. The System 336 fasteners have been installed in the unballasted tracks on the Budapest Metro and Turkish metro lines in Istanbul. The Bursa Metro will be equipped with W-Tram and in Ankara with system DFF21 fasteners.
Fig. 1 Fig. 2 Fig. 3 Fig. 4
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