Standardisation of embedded rail for light-rail systems

Embedded rail is used on street-running tramways. The rail section used generally includes effectively a continuous checkrail (known as the keeper) and enables a clear boundary between the paved road surface and the tram running rail.

Grooved rail is far from standardised. Twenty-six rail sections are defined in EuroNorms and seven of these are adopted across the eight street-running light-rail systems in the UK and Ireland, with some systems having more than one grooved rail section in operation. Embedded street running track represents a total of just over 200km of rail (100km of track).

This was the introduction by Professor Adam Bevan from the Institute of Railway Research at the University of Huddersfield and welcomed by UK Tram’s Centre of Excellence in October 2024.

Light-rail characteristics

Light-rail/tramway systems are generally characterised by sections of street running with embedded track and areas of reserved right of way where conventional ballasted track is usually employed. They typically consist of tight radius curves (down to 18 metres), steep gradients, light axle loads, small wheel diameters, low speeds (50-70km/h), and frequent starts and stops.

The vehicles are usually equipped with magnetic track brakes so that the trams can stop short of sudden obstructions and are fitted with sanders to overcome poor adhesion caused by traffic debris and other contaminants. An obvious risk arises if the groove becomes blocked. Further challenges exist when it is desired to run from the tramway onto heavy rail infrastructure (known as Tram-Train) which is beyond the scope of this article.

These conditions provide an arduous operating environment. The different light-rail networks are not homogeneous, with a large variation in operating conditions generally because of the need to adapt to local geography. Another factor has been that each system was developed separately, leading to a lack of standardised systems and components. There are short maintenance windows and, even though the tram service might be suspended overnight, it is not always possible to close the roads to other users. Embedded track is particularly hard and costly to replace, and the lack of standardisation often causes significant challenges when planning rail renewals resulting in higher costs and increased carbon footprint.

Due to the relatively small volumes, lead times on the procurement of grooved rails can be high as manufacturers wait for further larger orders to justify rolling a specific grooved rail section. These delays can allow rails to further degrade resulting in a potential safety risk and/or introduction of temporary speed restrictions leading to passenger delays and longer journey times. In addition, grooved rails are not currently manufactured in the UK and must be imported from Europe, meaning that the carbon footprint of transporting these rails to the UK is high.

Clearly, one way to prolong the life of the track is through efficient management of the wheel-rail interface. There are fundamental requirements to ensure that the wheelset fits the track, i.e., wheel flange width and height compatibility with the rail groove. Next is a requirement to manage the steering and lateral stability, something that is more challenging as many modern trams use independent rotating wheels. It is also possible that wheel-rail profiles chosen to manage stability and steering performance might generate higher contact stresses. This is something where vehicle dynamics simulations can be used to optimise conicity for a given system.

Standardisation

Back to the use of seven embedded rail sections. A DfT TRIG project (funded by the UK Government) was initiated in early 2024 aimed at identifying a grooved rail section which provides adequate performance across all UK and Ireland light-rail systems with on-street running. The project’s objectives were to:

• Define functional requirements and key performance indicators for grooved rails.
• Use a systems engineering approach to ensure all conflicting functional requirements are optimised.
• Develop a solution scalable to cover light-rail systems outside UK.

It is hoped that a standardised grooved rail section will increase availability and manufacturability of grooved rails in the UK, reduce cost of track construction and renewals, increase the allowable wear capacity before rail replacement, and optimise the wheel-rail contact conditions to improve performance at the interface.

Success criteria

Understanding what is required is always key to the success of any project and in this case the objectives were: ease of installation related to the depth of concrete troughs on embedded track sections; ability to pre-coat rails; ease of aluminothermic welding on site, with rail height, foot width, and longitudinal moment of inertia as key parameters.

The characteristics of the grooved rail sections were analysed, considering these objectives and key functional dimensions such as section height/width, groove width/depth, and gauge corner radius.

For conventional track, the maximum life of a rail is dependent on the amount of side and vertical head wear allowed. For embedded rail there is an additional factor. As the head wears, the depth of the groove becomes shallower, so the limit on vertical head wear is typically dependent on the depth of the groove and the maximum wheel flange height permitted. In some cases, there is an opportunity to machine the bottom of the groove to extend rail life, but that possibility is limited to rail sections with specific designs of width, depth and thickness of the keeper section.

All these factors were considered to achieve a longer life while ensuring that the structural integrity of the rail section is maintained. The candidate standardised grooved rail design is proposed to have adequate groove depth so that users would not need to use groove grinding.

Availability and cost (first and over the life cycle) are also important factors, so making sure that the weight of the section (i.e., kg/metre) is not excessive. An important factor related to the weight of the section and the cross-sectional area, is the second moment of area. The second moment of area of a rail is an important property used in the calculation of the rail’s deflection and stresses caused by a moment applied to the rail during the passage of a train wheel.

One of the most significant differences between the various rail sections in operation is the groove depth and width. As mentioned, groove depth governs the maximum level of vertical head wear before the wheel flange contacts the bottom of the rail groove. This is influenced by the wheel flange height which effectively increases as the wheel tread wears. Also, there’s a limit to how much side wear can be accommodated before there is contact between the flange back of the wheel and keeper of the opposite rail. This is also influenced by track gauge, wheel flange wear, and wheelset back-to-back dimension. Once keeper rail contact does occur, the thickness of the keeper section is likely to dictate the remaining life of the rail or when significant maintenance intervention, such as rail gauge corner weld restoration, is required.

As if all this wasn’t enough, the usual wheel-rail interface issues of wheel tread and rail head profile shape and rail inclination need to be considered. Just under 40% of embedded rail is vertical with the remainder inclined at 1:40. A similar proportion uses a rail head gauge corner radius of 10mm, with the remainder at 13mm. The interaction of the rail crown radius and wheel tread cone angle influence the guidance on straight track / shallow curves by generating rolling radius difference, while the interaction of the rail gauge corner and wheel flange root radius influence the guidance in sharper curves.

A gauge corner radius that is similar to the wheel flange root radius promotes two-point contact between wheel and rail leading to lower conicity, potential for higher flange wear, and stability issues at higher speed. Conversely, a gauge corner radius that is smaller than the wheel flange root radius promotes single-point (conformal) contact, delivering moderate levels of conicity, even distribution of contact across the rail, and good steering in moderate curves with controlled flange wear.

Several options for standardisation are being considered, including selecting an existing section which meets the defined criteria, optimising an existing design to meet the defined criteria, or designing a completely new rail section. Novel rail sections without the traditional rail ‘web’ and ‘foot’ are also being considered. However, initial discussions with key industry stakeholders suggests that utilising an existing design would be the preferable option. A candidate profile has therefore been recommended which promises to deliver on the objectives identified above.

Next steps

As with many research projects, there is more work to do before tramway operators can adopt a standard grooved rail. The project team is discussing findings so far with light rail operators combined authorities and other transport executives. It is also looking at opportunities to further optimise the candidate design, while assessing the economic viability of UK manufacturing which could dramatically reduce the carbon emissions associated with the transportation of rails.

Rail Engineer wishes them well in this initiative that should help to reduce light-rail costs.

Colin Robey, chair of UK Tram’s Centre of Excellence, observed:

“This is just the sort of project that the Centre of Excellence is keen to support. Unlike the Heavy Rail industry where everything is required to conform to national standards, Light Rail has a number of areas where standardisation is difficult because of slight differences between systems. In correspondence with German Rail Engineers some of their tramways face the same problem as the UK in that they are all different and not standardised.

“Grooved Track form is therefore the ideal area for the industry to cooperate through such standardisation and this study will allow us to start to move to a much more cost efficient and effective way of procuring embedded track for both new systems and existing system renewals and potentially reduce the lead time on a specific product. There are also non-technical areas that will require consideration such as integration between the system owners and maintainers through the Light Rail Engineers Group and the Local Authorities Procurement Regulations but the case for change should highlight the benefits.

“UK Tram and the Light Rail Safety and Standards Bureau (LRSSB) have been involved from the start with UK Tram’s lead engineer Phil Terry and Craig O’Brien from LRSSB monitoring progress.”