HomePermanent WayEven better than the real thing?

Even better than the real thing?

Synthetic sleepers have been in production for over 40 years, but it is only in the last 20 that European infrastructure managers have adopted the technology. With a contract to supply FFU timber for the Chelsea River Bridge refurbishment, leading synthetic material manufacturer, Sekisui, had an opportunity to showcase its product on a project that made use of all its advantageous properties.

Constructed in 1863, the Chelsea River Bridge – also known as the Cremourne Bridge or the Battersea Railway Bridge − links Battersea to north-east Fulham and carries 18 trains per hour, including passenger services and freight.

The bridge has seen considerable refurbishment work over the past decade. This year, Sekisui was chosen by Network Rail to provide synthetic rail sleepers for its renewal of the bridge’s track. More than 120 fibre-reinforced foamed urethane (FFU) longitudinal bulks (w/h/l = 450/130/7,500mm) were installed on the existing bridge structure, on top of which over 800 FFU bridge sleepers (w/h/l = 240/130/2,600mm) were placed. The pre-fabricated FFU longitudinal baulks and bridge sleepers were pre-drilled before shipping at the production plant in Japan. Despite all the recent challenges with Covid and shipping, this project was a great success.

Chelsea bridge – cross sleepers from FFU. Photo credit: Günther Koller.

A long history

Synthetic sleepers aren’t exactly a new phenomenon. Sekisui began developing its product in the 1970s, after Japanese National Railways found that its wooden sleepers were degrading after exposure to the elements. A longer-lasting material with the same characteristics of wood was called for and Sekisui’s FFU timber fit these requirements.

“Some 1,550 km of railway was laid on modern FFUs, globally, between 1980 and 2020,” says Sekisui’s Patrick Childs.

 “Many have been in service for 35 years and their use has been analysed by the Technical University in Graz, Austria and the University for Technology and Architecture in Frieberg. These sleepers are of similar weight to our wooden ones and use the same types of fastenings, and so on. The initial trials in the 1980s were carried out using bi-block FFU sleepers incorporated into a ballastless track area in a tunnel.”

Sekisui’s FFU was first used in Europe by the Vienna public transport system, Wiener Linien, on the Wienfluß Bridge project in 2004. Since then it has been used across Europe, specifically on metros and light rail networks in Hamburg, Berlin, Düsseldorf, München, Bochum and Toulouse. The London Underground and the Paris and Lille metros have also installed these artificial sleepers.

Ashford bridge project. Photo credit: Günther Koller.

Multiple benefits

The benefits of FFU are manifold and it’s no surprise that it was chosen for use on the Chelsea Railway Bridge. The material has the weight of natural wood and many of the same properties. Its close resemblance to traditional timber means that the visual appearance of the bridge remains intact. It is also easy to repair and, most importantly, has an extremely long life.

In 1996, Japan’s Railway Technical Research Institute (RTRI) carried out tests on sleepers which had been installed in 1980. Several fatigue tests simulated 100 million load cycles. These tests led to the prediction that FFU would have a life expectancy of 50 years. In 2011, sleepers from 1980, which had now been in service for 30 years, were again removed and tested by RTRI. As a result, RTRI confirmed that these FFU sleepers would work safely for the next 20 years.

FFU is not affected by UV light and retains its technical properties after many years of exposure. Where FFU is not painted, long-term UV irradiation only leads to discolouration of the surface. Not only does this increase the material’s longevity but also has environmental implications as FFU products do not need to be treated with harmful chemicals to protect them from UV exposure.

Ecological advantages

Continuing on the environmental theme, FFU has been proven not to contaminate water sources, as reported by the Japan Food Research Laboratories’ 1994 report, OS57110607-1. The organisation conducted water quality tests with FFU synthetic wood and concluded that the water into which FFU was immersed met the same quality standards as control water. A further environmental benefit is that FFU is 100% recyclable.

Currently the UK’s record in recycling, especially plastics, is far from good. Perhaps we should consider the wider use of what are claimed to be recyclable synthetic railway sleepers with a proven 35-year

durability? This would surely be beneficial due to their long life on bridges as waybeams, in tunnels, and to carry switches and crossings.

The use of recyclable synthetic sleepers is ramping up across the UK rail network. FFU synthetic sleepers were first installed by Network Rail on track and bridges in Ashford, Kent, in 2014, followed by bridges in Rochester and Faversham. In 2019, the diamond crossing in Newark was replaced using FFU sleepers with a size of 16m in length, 70 cm in width and 38 cm in height.

A 2013 EU-wide creosote ban, which exempted railway sleepers until 31 July this year, has now come into force and the alternative material − hardwood − is mainly sourced from Brazil and is not sustainable.

RTRI Test values for FFU sleepers new, after 10, 15 and 30 years in use. The values are the material failure values.


FFU sleepers can be worked and machined in much the same way as traditional timber, using standard tools and existing fastening devices. These properties are a result of the material’s manufacturing process.

“The synthetic sleepers are manufactured using the ‘pultrusion process’ in which glass fibres are soaked and mixed with polyurethane, then hardened at a raised temperature, moulded and cut to length, says Patrick. “Currently, the maximum practical length is just 10 metres due to shipping restrictions. The FFU bearers for Network Rail’s renewal of Newark Flat Crossing were made in Japan, shipped to Britain and are up to 8 metres long having been made using 30mm thick layers of material.”

Various manufacturing processes can also be applied to suit the requirements of specific locations. Examples include cutting the material to produce the correct angle to match the designed cant of track, drilling screw holes, milling out spaces for accommodating other parts of the structure and surface sanding. The technology offers the advantage that nearly any shape of sleeper can be precisely produced from a plan – for example, soleplates from a height of as little as a few millimetres up to sleepers of 16 metres length, 70cm width, and 38cm height.

Ashford, run on/off and ballasted track between two bridges. Photo credit: Günther Koller.
Chelsea bridge. Photo Credit: Network Rail

Each sleeper is also given a unique identification to ensure it is laid at the intended location on site. All of this means that sleepers can be delivered to the project site ready for installation. This ease of use can lead to much shorter possessions, saving time and money and minimising disruption.

Considering the advantages of the technology, along with the rail industry’s increasing awareness of environmental matters, synthetic timber looks to play a major role in the future of rail construction. To find out more visit www.sekisui-rail.com.

Special thanks to Peter Blakeley and Christopher Brown from Network Rail for all their hard work and support in making this project happen.

Lead photo: Newark Crossing, 8 baulks from FFU (w/h/l = 700/380/16,000mm). Photo credit: Günther Koller

Rail Engineer is the leading independent quality monthly magazine for engineers, project managers, directors and leading rail executive decision makers. Head to www.railsubs.com to make a free subscription to RailEngineer magazine or one of its sister publications.


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