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Rail masterclass: Second chances count

An automated submerged arc welding machine undertaking gauge corner restoration.

The operating environments for tram and light rail systems pose interesting challenges around rail maintenance, so to explore contrasting philosophies we must first explore the rail structure and its functions.

Grooved rails are used wherever rail and pavement, or indeed roadway, meet. Where the route is shared by pedestrians and/or road vehicles, grooved rails provide two key functions combined into one item. The first is the rail head, which supports and steers the tram passing over it just like a ‘standard’ rail. The second is to provide a protected flangeway for the vehicles’ wheel flanges to run unimpeded. Asphalt road surfaces are more flexible than many would imagine and without being restrained during use and seasonal changes, the road surface could end up obstructing the flangeway, leading to obvious safety hazards.

The ‘keeper’ part of the rail ‘keeps’ the road surface at a safe distance from the rail head preventing the ‘groove’ from becoming obstructed. The head and keeper are also typically at very similar heights to provide the minimum impact to other road users, although the gap between the two can pose a challenge where cyclists travel along the same route.

The complex shape of grooved rails makes production challenging and so the number of global suppliers is much smaller compared to conventional rails. When installed, buried to the top of the head and keeper, grooved rails pose interesting rail inspection and maintenance issues too.

Tortuous track alignment

Typically, tramway and light rail systems follow existing roads through established cityscapes. As such, curves are often tight – sometimes very tight (you might even call them corners); 25m radii are not uncommon and many traditional railway engineers might ask if you had missed a zero from the drawing! In shared streets, the use of rail cant – a banking of the curve sometimes called superelevation – is not easy, if indeed possible at all. This typically means tight radius curves have zero cant. Both factors give the rail (and indeed vehicle) designers some interesting headaches in minimising the acceleration of both rail and wheel wear.

Many cities have grown up nestled between hills in the local landscape. Famously, Sheffield was founded upon seven hills and the presence of such changes in elevation poses real challenges as rail vehicles are not naturally good at climbing or descending steep gradients, requiring more tractive and braking effort, meaning wear rates that are typically higher than for mainline railways.

Once again, many traditional rail engineers would be looking for a misplaced decimal point as gradients of 10% are not unheard of.

Aggressive environment

Embedding rails in the road surface also causes environmental challenges as the groove sits below the road surface and so collects water, dirt, debris and litter from the road surface. In some climates roads and footpaths are salted, sanded or gritted in winter to melt ice and provide other road users with enough traction to continue their journeys. Grooved rails act as a natural trap for such materials so drainage is a key factor when the network is built as the presence of such debris at the wheel/rail interface can increase wear rates by acting as an abrasive between the two.

Both alignment and environment contribute to accelerated rail wear, but this wear is not equally distributed around the rail. As vehicles negotiate sharp zero cant curves they carve out the side of the rail which is known as side wear. This is probably the most familiar to many, but both keeper wear and head wear can limit the useful life of the rail.

The keeper which usually has the job of keeping the road surface away from the groove can also act as a ‘check rail’ restraining the flange back of the wheels, providing additional guidance around sharp curves. The author does not endorse this practice for several reasons, but also acknowledges it is an accepted standard practice in many networks.

So we have established that grooved rails can be subject to high wear. We now explore how high, and what can we do about it?

For the most part, the highest-wearing areas of any track system are the sharpest curves. Let us take as an example a 25m radius curve and an R260-grade grooved rail. Depending on the vehicle type, an operator could be looking at 9mm of side wear per year on an intensively-used system, equating to a rail life of under two years if repairs are not carried out; this could be even shorter if less wear-resistant rails are used.

The fact that grooved rails are embedded in the road surface also makes their replacement a complex, disruptive and very expensive task. In fact, it may cost five times more to replace a grooved rail than a flat-bottomed (Vignole) rail. Closing the route to all traffic (road, rail and pedestrian) in the middle of a densely-populated city is hardly going to be popular, but is necessary as intrusive methods are required to break the rail free compared to traditional trackforms, which adds time, cost and noise nuisance. So, there are obvious very clear drivers to maximising the life of the rail. There are several different schools of thought on just how to approach this problem:

1.  Do nothing – replace rails as required

2.  Apply wear-resistant weld deposits to wear-prone areas

3.  Use more wear-resistant rails

4.  Weld repair rails in situ

Each option has its merits and drawbacks and there is no ‘best’ answer as each situation depends on many factors. The author favours an approach that leads to lowest total lifecycle costs, but many projects are not procured with this as a priority.

Comparison of wear rates and hardness for different grooved rail grades.

1. Do nothing – replace rails as required

Although this approach may sound unwise, it does have some benefits as in the first instance it minimises and simplifies the initial installation/project costs. This often makes it the default solution where the construction contractor does not go on to own and maintain the asset, and build costs are a priority.

Essentially it trades lower capital costs for higher future maintenance expenditure. Once a system is operational, regular measurements can be taken to allow wear rates to be calculated. This means replacement schedules can be planned well in advance, minimising their impact to some degree. Also, an often-unquantified benefit is that because this approach necessitates the regular replacement of rails on tight curves, it also limits corrosion of these areas, limiting this difficult to predict and manage degradation type.

2. Apply wear-resistant weld deposits to wear-prone areas

Before embedded rails are installed you can apply a hardened layer to wear-prone areas, sometimes referred to as hard facing.
This is done by machining or grinding away the rail surface, installing a wear-resistant weld using a special alloy and reprofiling the rail to the original profile. This process is completed in a factory environment to give a high quality weld and requires a high pre-heat temperature not achievable in a street environment; it is a slow process and therefore costly too, but it does increase the life of the rail significantly.

3. Use more wear-resistant rails

This philosophy will be recognised by those familiar with the heavy rail industry where many strive to use more wear-resistant steels to increase rail life. It seems obvious that if you use more wear-resistant rails then life is increased, however other factors often come into play such as rolling contact fatigue (see Rail Masterclass – TAUT 991).

Moving from an R200 grooved rail to an R260 grade improves the wear resistance (and therefore usually the rail life) by a factor of around three times. Grades such as R290GHT and R340GHT can increase this still further. There are two main methods of making rails harder and more wear resistant: you can vary the composition of the steel (alloying), or you can accelerate the cooling of the rail (heat treatment). Heat-treated grades use the suffix HT. This article does not try to cover every grade available from every manufacturer but shows some typical examples.

4. Weld repair rails in situ

In a similar way to weld repairing using hard deposits, processes exist to repair rail side wear while they remain in the road surface. Often called ‘Gauge Corner Restoration’ (GCR), this takes several different forms but has undergone developments in recent years to offer improved alternatives to traditional techniques.

So far we have only discussed the ‘First-life’ of a rail, i.e. the time or traffic until wear reaches a limit where intervention is needed. However, if you can weld repair rail back away from such limits safely then you give the rail a second chance or ‘second-life’. As this process is generally repeatable, so long as the rail remains in good condition and the head wear is not excessive, a rail may be repaired perhaps six or more times giving it multiple lives. Although focused on side wear, this process can also be used to prevent excessive keeper wear. Weld repairing the gauge corner of one rail prevents keeper wear of the other as the distance between wheels is fixed.

For weld repairs to be successful, two different and contrasting approaches tend to be found. The first is to use a rail with a lower carbon content: generally higher carbon steels are more wear resistant but are also harder to weld – although there are important exceptions. The second is to utilise clever metallurgy and advanced welding techniques to help make steel grades that are both wear-resistant andcan be welded in situ.

The first practice leads to the rather counter-intuitive situation where some tramways utilise softer rails on their tightest curves. They do this in the belief that they can undertake weld repairs in situ more reliably to deliver longer lifetimes than a non-weldable rail would deliver.

More recently, the second option has been championed by some manufacturers who have introduced grades specifically aimed to deliver both a weldable and a wear-resistant rail, although their approaches to the metallurgy are quite different. For example one uses lower carbon grades and heat treats them to higher hardness levels. Another uses advanced metallurgy compared with weld repair processes to deliver a naturally hard but weld-repairable rail steel.

One important factor, which many networks learn the hard way, is that all welding done in-track carries some level of risk. No matter how much you control the process failures will occasionally occur, especially when producing long lengths of weld material around entire curves. Network attitudes to this vary considerably. Some European systems use very basic manual stick weld repairs, accepting the risk that they will need more frequent repairs, while others are more risk averse and drive for a highly-controlled and reliable, more sophisticated weld repair such as those produced by automated submerged arc welding machines.

Those networks that accept and manage the risks of gauge corner repair welding can extend rail life so that it is no longer determined by side wear, instead being dictated by rail head wear.

Strategy comparison

Let us look at how these philosophies compare throughout the life of a fictitious rail curve using a heavily-trafficked 25m radius curve as a harsh but reasonably common case study. Some assumptions are made to simplify the examples. It has been assumed that the rail head wear safety limit is 14mm, the side wear limit is 13mm and when we consider strategy number four, any weld repair has the same properties as the original rail. The last of these assumptions may not be 100% accurate, but simplifies our calculations significantly. For the purpose of the comparison it only affects the number of weld repairs required, not the overall rail life as that is governed by head wear.

Strategies 1, 2 and 3

When looking at the three strategies in isolation the ‘hard-faced’ rail achieves the longest life. It should come as no surprise that this method is well-embedded in the industry. However, it is also by far the costliest and most time-consuming as the rail must be specially prepared and welded in a factory before being installed. The next best choice for a long life is to use more wear-resistant rail grades; these are now often cost-effective enough to consider their usage on an entire system rather than just the sharpest curves.

Adding strategy 4 – Second chances count

The implementation of GCR as standard practice on a tramway network changes the picture enormously, but it must be noted
that not all rail steels are repairable in situ. Factory hard-faced rails and the hardest heat-treated grades for example are generally unsuitable.

The use of weld repairs on compatible wear-resistant rails can transform the life of the rail from less than three years to over 23. Whilst any weld repair process is not risk-free, the benefits to networks can be colossal both in financial terms, but also to minimise network disruption caused by rail replacement. It is no surprise then that much development work has been carried out in this area by both rail manufacturers and service providers alike to provide rails compatible with in-track GCR repair and to optimise these repair processes.

Which strategy is best?

The million dollar question! While the author promotes the use of the lowest lifecycle cost options for rail, it must also be noted that due to various and sometimes complex financial drivers, this is not always shared by all tramways. So when considering which is the ‘best’ option, the type of project and its funding mechanism often affect the answer.

If the question is phrased as ‘Which is the lowest installed cost?’, then you are usually looking at most basic lower grade rails – e.g. R200 – trading a lower capital expenditure for higher maintenance and replacement costs.

Rephrasing again to ask, ‘Which rail results in the lowest maintenance costs?’, then you may well choose the hard-faced option to minimise repair operations while increasing the rail replacement budget.

If the question asked incorporates installation, maintenance, and replacement costs, then often the time horizon used for the calculation can be important in favouring one grade over another. In the example above you would likely get different answers for a ten-year horizon to one that looked at 20 years or more.

Lastly, in a world where European steel and rail producers are often under financial pressure, the answers may be dictated not by any strategy and budget, but simply by which rails are available for the project at any given time…

Article appeared originally in TAUT 994 (October 2020)