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Track management: repair or replace?

On-street tramway construction must take into account future maintenance. Birmingham city centre in July 2014. (Image credit: T. Sullivan)

Originally conceived in the 1850s, the concept of the grooved rail uses a chunky rail head that lies flush with the roadway to carry a tram’s tyres while a groove accommodates the wheel flange. A check, or keeper, forms the outside of the groove that the wheel flange slots into to steer the tram.

Grooved rail requires a more substantial mounting for weight transfer, while steel ties at regular intervals maintain the gauge. Improved steel grades and alloy compositions in recent decades have seen significant enhancements in rail life, evolving
alongside advances in rolling stock technology that have seen trams get longer, heavier and with higher axle loads. Likewise, direct contact with concrete and asphalt in the early days of tramway construction has evolved into advanced embedding using polymers and foams that reduce vibration and damage to the surrounding highway infrastructure and help mitigate issues such as electrolytic corrosion.

This rail embedding system is at the heart of the street-running tramway. Most are of a resilient polymer type, with variants including bonded cork materials poured into a channel surrounding the rail before setting, and recycled rubber rail seats. As new track is often installed as part of ‘turnkey’ projects, the choice of system usually rests with the contractor rather than the engineer who will have maintenance responsibility for the decades to come. Factors influencing the choice of rail embedding include:

  • Traffic: Heavy road traffic calls for substantial support. High lateral loads from heavy vehicles turning, accelerating or braking over tramlines can make rails roll, damaging the reinforcing material and placing incredibly demanding conditions on the rail. Concrete laid up to the top of the rail is also more supportive than asphalt.
  • Insulation: Good electrical insulation is vital in mitigating stray current issues that can affect sensitive under-street utilities apparatus and also in the prevention of electrolytic corrosion. Sensitive structures such as laboratories, hospitals, concert venues and radio stations also call for low levels of environmental noise and vibration in their immediate vicinity – this can require additional dampening mechanisms.
  • Adhesion: Pre-coated polymers can be extremely difficult to remove, negating any scrap value or transfer of used rail, so the ideal system will be easy to install and remove.
  • Resilience: Resistance to wear and chemical attack is vital. Likewise, water ingress into any elastomer is bad and as ice it can seriously degrade embedding materials such as foam.

Wear and tear

Economic pressures have seen a shift towards lifecycle cost reduction as the last things any undertaking desires are costly and disruptive programmes of premature rail replacement. Severe rail wear causes uncomfortable passenger journeys, increased vehicle maintenance and, in the most extreme cases, potentially serious safety issues.

One of the most challenging issues for the tramway engineer is gauge corner wear. On the tight curve radii, sometimes as low as 15m, that is an unavoidable consequence of street running in tight urban environments, lateral forces from a tram’s wheel flange create high and rapid levels of wear on the side of the grooved rail’s head. As this wear increases, the outside edge of the wheel flange approaches the check on the opposite rail which can become very thin and even sharp. This issue is exacerbated if the supporting bed allows the rails to roll and is even worse if there are no tie bars.

Tramway and light rail networks set limits for allowable side wear for plain line running with additional limits for check wear to ensure the integrity of the rail for safe operation. Acceptable wear limits are the minimum section dimension at which the rail is capable of withstanding the stresses of passing vehicles without breaking or deforming. Each system has different maximum limits and there are legitimate reasons for this, such as varying rolling stock specifications, axle loads and wheel profiles, different rail support structures and deemed acceptable levels of flange running. Once wear reaches these limits, the rail has to either be repaired or replaced.

Degradation depends upon many factors including the relative hardness of the wheel and the rail, the choice of rolling stock and its upkeep, and the maintenance regime for the track system. Gauge corner wear usually governs the life of an entire piece of track, and replacing the rail – with all the issues associated with street closures and breaking up the roadway – can be very expensive.

The good news is that its onset can be delayed by lubrication or friction management of the wheel/rail interface in curves, either from wayside top of rail applicators or vehicle-mounted units. Repairs involve weld restoration of the corner gauge, but this can be costly and requires skilled attention to ensure a robust weld and a process that avoids damage to the surrounding embedding material.

Building up the parent metal with weld is an attractive solution, but traditional rail welding standards call for the heat-affected zone – the weld plus 75mm either side – to be pre-heated to over 300°C. As embedding materials can fail at these high temperatures, to address the issue, a technique pioneered
in the UK almost a decade ago on the Sheffield Supertram network saw the development of ‘cold welding’. This requires a pre-heat of just 60-80°C. The secret to this technique is that heat developed laying down the second weld bead is used to heat-treat the root bead, and so on. The final sacrificial bead is ground off. The lower temperature weld avoids damage to the surrounding polymer material while maintaining robust, crack-free weld deposits.

After repeated repairs tramway maintainers face the dilemma: repair or replace? The installation of hardened rail with a high wear resistance – either through a higher carbon content or heat treatment – can be attractive as, in general terms, the harder the rail, the longer it will last. But this option comes with caveats.

More common ‘softer’ R200 and R260 grades provide better welding and deposit-welding conditions on embedded track, and allow easier rail reprofiling through grinding or milling. If the main objective is to avoid rail replacement at all costs, this may offer the best solution to keeping systems open for business for longer. Rail/wheel interaction will by itself help regular grinding and maintenance of the rail to avoid surface micro-cracks.

On the other hand, it is natural that more specialised rail comes at a higher initial cost, also bringing with it increased challenges in future maintenance as both types are more difficult to affect future repairs. Joining heat-treated steel to non-heat-treated rail can also result in inconsistent properties across the joint, or a requirement for more expensive composite welding processes.

Increasing carbon quantities has been found to increase resistance to wear and rolling contact fatigue, so judicial alloying is necessary to limit inherent brittleness. Some recent data also suggests that rails of more than 300HB hardness may suffer more adversely from ‘squats’, leading to more frequent grinding as well as a shortening of the rail’s overall lifespan.

Squats are a type of metal fatigue that result from wheel impacts that ‘bruise’ the rail head. As with other phenomena, squats deteriorate quickly if left un-remedied and, if unmanaged, can lead to rail fracture. So although the periods between weld restoration, grinding and milling may be longer, the rail may need to be replaced once those wear limits are reached. So perhaps harder and harder rails are not the full answer.

Maintainers and manufacturers seek the holy grail of smooth riding characteristics, minimised rolling noise and vibration, long intervals between repairs and easy weldability. Sheffield Supertram, Lyon and Casablanca have all undertaken either rail replacement or new-build installations in recent years with these so called ‘high-performance’ rails, and, so far, with encouraging results.

Meanwhile, modern switch devices and turnouts promise to be ‘lubrication and maintenance free’, providing benefits for the environment as well as cost-savings. Modular grooved rail turnouts on precast concrete slabs provide a fully-assembled unit with integrated insulation and continuous elastic bearing, while additional options include pre-assembled setting systems and drainage boxes.

Corrugation and friction modification

Rail corrugation is a more complex challenge. While experts still argue about its exact mechanics, most agree that it results from the interplay between the natural frequencies of the elements involved – the vehicle suspension and the rails themselves – together with complex Hertzian forces at the wheel/rail interface. At best it causes the annoyance of noisy (or ‘roaring’) rails, at worst it can significantly reduce rail life and cause vehicle damage.

As corrugation begins with a discontinuity in the track system, when installing or replacing rails it is important to ensure that welds are accurately trimmed and that support systems are continuous. What seems certain is that corrugation is more prevalent in embedded track than ballasted track.

The usual means of rectification is to grind the head of the rail, but height is lost each time this happens and repeated treatments reduce the rail’s cross-section, eventually creating a ridged tripping hazard for cyclists and pedestrians as the running surface sinks lower into the road.

Corrugation is a ‘catch-22’. If left, it gets worse at an exponential rate, yet conversely, rail life is reduced each time the rail is ground. A possible solution to early rail replacement may be to build up the rail head with careful welding – perhaps using a modified version of the cold welding technique described above to reduce the potential for damage to the embedding material.

Friction modification is a promising method of slowing the onset of corrugation. Water- and chemical-bound solutions are available to reduce the friction coefficient at the wheel/rail interface and where used it has been noted that noise levels fall appreciably. While some may argue that wheel spin and locking of the wheels under hard braking must necessarily increase if friction is reduced, there seems little doubt that, whatever the mechanism, friction modifiers for street tramways show benefits for both the environment and maintainers alike. These concerns, however, must be balanced against the likelihood of additional wheel burns and flats.

Real-time digital monitoring technology is increasing the visibility of track asset condition, offering new possibilities for predictive maintenance. While such systems are still relatively new in light rail applications, the potential for long-term cost savings could be significant in reductions to physical inspections and generating a more accurate picture of an overall network’s health.

No more rusty rails

Electrolytic corrosion is another problematic issue. The best safeguards are to ensure the embedding material on street-running sections is insulating and continuous throughout, the drainage system is effective, welds are sound and all joints are well bonded.

At its simplest, electrolysis occurs when direct electric current is passed from one pole to another through dirty water. As the water splits into its component elements – oxygen and hydrogen – the former collects at the positive pole (anode) where it is incredibly corrosive. If the rail insulation fails at two or more points then there is conductivity outside the insulating sheath and the current will pass down two parallel paths – the rail itself and outside the sheath. The latter will usually have a much higher resistance and therefore carry a small proportion of the current, but the rail at the positive end is an anode and a small current over a period of many days or weeks can be extremely damaging.

Modern adhesive rail coatings can help prevent rail corrosion, usually through a simple one-pack application that is resistant to both abrasion and mechanical damage. Although such coatings shouldn’t impede welding of the rail, they are nevertheless designed to be easily stripped back to allow weld restoration and reapplication.

Thanks to Paul Baker of Bakerail Services and Peter Daly of Thermit Welding in the preparation of this article.

Feature originally published in June 2017 TAUT (954).