Full speed ahead - challenges for the UK high-speed railway project

Materials World magazine
3 Jan 2011
speeding train

As the UK embarks on an ambitious concept for a high-speed rail system,
Professor Roderick Smith of the Future Rail Research Centre at Imperial
College London, explores the challenges ahead. 

The building of a new UK high-speed network will be a long-term project but one that will eventually link all the major cities in the UK via 1,300km of dedicated high-speed track. The country’s motorway system, built over the last 50 years, represents less than one per cent of the road network, but carries 20% of all passenger traffic and over 40% of freight. The existing rail network is four times longer than this but carries only eight per cent of passengers and 12% of all freight. Therefore, the case for a high-speed rail network is as much about capacity as it is speed.

Train of thought

Although the building of this network will be done primarily by civil engineers, it is important to remember that high-speed lines operate as a system, with the vehicle and infrastructure matching like a well-fitted glove.

For example, tunnels on such a system need to be of large diameter to accommodate the pressures generated by high-speed. More energy is needed to push the train through the tunnel and the costs are increased. The pressure changes cause fatigue loadings in the vehicle shells, limiting their useful lifespan, and the exit of the train can cause a sonic boom, which can spread over a wide area. The trade-offs between these factors necessitate a wide thinking approach.

The fatigue design of carriages with air tightness as its goal is a considerable challenge. It is likely that aluminium will be the major material of construction and great attention needs to be given to its forging, usually in 24m long carriage length extrusions, fabrication and welding. The air tightness needs to be verified by full-scale testing of whole carriages and a sufficient fatigue life established by simulating the pressure changes during the passage through a tunnel.

When trains are running on the new route, the maintenance costs will be a substantial part of the overall operating cost. Some form of slab track will probably supersede the use of ballasted track. Furthermore, the train will have to be as light as possible with uniform axle loads arising from distributed traction. Lightweighting calls into play sophisticated material selection and the need for a better understanding of the dynamic loads at the wheel-rail interface so that adequate structural integrity can be managed.

Travelling light

As an order of magnitude, many of our existing trains have an overall structural mass of around 1,000kg per seat – the world’s best high-speed trains are around half this value per seat. To achieve this goal, every item has to be examined and redesigned for lightness. Interior fittings and seat shells can be made from composites and plastics, mirroring the developments that have taken place in aircraft (the total mass per seat of a Boeing 747 is about the same as a high speed train).

Similarly, floor panels and internal wall coverings must be slimmed down whilst fire resistance and vibration damping capacity are added to the shopping list.

The best distribution of weight on the axles comes from dispensing with the traditional concentrated power car and having each axle powered by its own motor called distributed traction. This equalises and minimises the mass supported by each axle and additionally gives much improved traction characteristics. Electric motors have hugely improved in both mass and volume density largely because of improvements in the magnetic properties of the key construction materials. One kilowatt per kilogramme is now possible.

The axles and wheels comprise what is known as the unsprung mass below the main vehicle suspension and in direct contact with the rail. Lowering this mass is key to reducing the dynamic loads of the train/track interaction. Because the wheels and axles themselves are directly subjected to these dynamic loads and any failures can lead to disastrous consequences, it is the classic compromise of the need to reduce mass whilst maintaining structural integrity through wear – in the case of the wheel tread – fatigue for both wheel and axle.

Remarkably, although the fatigue of axles was the catalyst for the first systematic studies of fatigue way back in the 1840-65 era, considerable difficulties remain in quantifying the complex stresses to which these components are subjected. Whilst recourse can be made to finite element stress analysis, direct experimental measurement of the loads in service and the identification of critical locations are still necessary and this data is often poor.

Wear and fatigue

The classic problem associated with railways is that of deterioration at the wheel/rail interface. The passage of each wheel over the rail is an irreversible event causing both wear and fatigue.

If the wear is high, cracks are rubbed out as they initiate: if the rail is too hard, fatigue cracks grow. A huge research effort has been made in this area and although solutions have been found, quantitative understanding based on the physical understanding of the processes involved is still lacking.

Wrought and cast iron were replaced by steel early in railway history and cumulative improvements in steel making over the last century have increased the life of rails, but further progress must be made to accommodate the increasing stresses of high-speed performance.

Great changes have been made since the birth of the railways – the lightweight mechatronic train has replaced the heavy mechanical steam locomotive, yet many challenges remain. Railways have throughout their long history been symbiotically dependent on progress in materials and this will continue into the future. However, a new high-speed train network will place additional calls on the electricity generation and distribution capacity. Therefore, it will also be urgently necessary to replace hydrocarbon fuel and extend our generation capacity without increasing carbon output.

The building of this type of railway offers the opportunity to break free from the constraints of the UK’s historic past and the materials community has a key role to play in achieving new levels of safety, reliability and punctuality.

It will also offer great opportunities to the UK construction industry over a considerable time period, and, if the plans are properly integrated with regional development, the dynamic of the nation can be transformed by improved connectivity. It will therefore need to draw on a corpus of expertise, which has sadly diminished in the UK since the railways were privatised, but now needs to be renewed well in advance of detailed planning and building of the system.

The generation of people with high level technical rail expertise has not been a major concern for existing privatised railway. The new railway will depend on highly skilled, highly motivated and flexible staff, and steps need to be taken now to ensure such people will be available.

The long proud history of the railways in the UK must not be allowed to wallow in a sea of hubris. The UK’s railways have long dropped out of the premier league and we are well behind other countries in developing high-speed. In China, trains travel at 350kph in everyday service. Over 7,000km of high-speed railway has been opened there in the last two years – another 5,000km will follow soon. The question now should be is the UK’s ambition to open just 200km by 2026 sufficient?  

Brief Timeline: Railways & Materials

1812 Steam locomotives introduced, Middleton Railway Leeds.
1830 First inter-city railway, Liverpool and Manchester Rails cast wrought iron, carriages are made of iron under-frame and the superstructure is wooden
1859 First steel rails introduced
1863 First underground railway in London – steam-powered
1900 Electric traction introduced
1913 First diesel railcar in Sweden
1933 Aluminium carriages at Chicago World Fair
1930s Streamlining era with steam speed record from the Mallard 203kph
1950 Aluminium carriages
1960 Steel replaces wood for carriage superstructure
1960s Welded rail introduced and bulk steel making improved with lower inclusion count
1964 High-speed rail introduced in Japan and lightweighting becomes increasingly important
1970s Wooden sleepers replaced by steel, concrete and composites. Long aluminium extrusions of 6xxx series alloys for carriage manufacture and integral monocoque construction of steel carriages without under-frames
1980s Increasing use of plastics and, later, composites largely for interiors
1980 Concrete slab track
1990s Trains become increasingly mechatronic. Energy absorbing crashworthy design of rail vehicles. Friction stir welding assists aluminium vehicle manufacture.
2000s Rapid expansion of high-speed rail worldwide, energy and emissions become important considerations. Costs become an overriding concern.
2007 World speed record set in France at 574.8kph

Note: The time from introduction of a technology to widespread use is variable and rather long for the railway industry. Typically trains have a 40-plus year life and infrastructure more than 100 years. These long technology windows mean that rail has traditionally been slow to adapt to change.

Further information

Professor Roderick A Smith, Imperial College London, SW7 2BX, UK. Tel: 020 7594 7007. Email: roderick.smith@imperial.ac.uk Website: www3.imperial.ac.uk/merailways