INTRO: Raising the maximum speed on conventional lines to 225 or 250 km/h is now feasible, depending on investment resources, the technology available and the legislative background. Much of the technical knowledge is in place to make the required advance, but translating it into a coherent overall package is proving a considerable and as yet unsolved challenge
BYLINE: Richard Gostling
Director, TechnologyAEA Technology Rail
The construction of dedicated infrastructure for speeds of 200 km/h and above has been the watchword of railway progress in the past decade. Although less glamorous, achievement of higher speeds on conventional lines presents many significant commercial opportunities at a fraction of the cost of building new lines, and is likely to become the goal in the next decade. Although the speeds involved are lower, the overall technical challenge is in some ways more difficult, and attempts to break through the 200 km/h barrier on conventional lines are so far limited to TGVs running at 220 km/h on parts of the Paris - Bordeaux main line.
The conventional railway has progressed mainly in a series of evolutionary developments almost since its inception, always maintaining backward compatibility. A major step change was the introduction of 200 km/h operation in the 1970s. Now a further step is required, and the challenge is to achieve it within the parameters of the conventional system.
The railway is a technically integrated system, and every one of the many technical issues must be identified, understood and overcome. Failure in any one area results in failure of the whole. In many cases, the background technical knowledge to achieve the step forward is already in place, but a concerted effort will be needed to combine all of this knowledge into one effective, reliable, operational package.
The challenge is particularly relevant to countries like Great Britain, with significant plans to upgrade existing routes.
Ride comfort and tilt
Quality of ride is the issue most normally associated with high speed on existing lines, and the numerous examples of tilting train design throughout the world illustrate how much attention has recently been paid to maintaining comfort whilst traversing curves at higher speed. Steady-state curving is, however, only one part of a complex issue, and not even the most critical.
The ultimate speed limitation is usually to do with irregularities in curves, in particular the transition which even when perfectly maintained provides a roll input to passengers which cannot be compensated by a tilting system. The fundamental understanding of the human response behaviour was developed many years ago by BR Research (now AEA Technology Rail) using the Advanced Passenger Train, when the current design limits were established, and the magnification in discomfort for a given irregularity when positioned on a curve were first established.
Early work on tilt attempted to provide full compensation for lateral accelerations but this was shown to be inappropriate, as the acceleration and visual inputs to passengers need to be consistent. Passenger comfort perception tests demonstrated that partial tilt with around 75% compensation is close to the optimum and also permits use of a larger body profile.
Even on straight track, ride comfort is an issue, as longer wavelength track inputs are transmitted through the lateral suspension. For example, on the British MkIII coach the lateral suspension frequency is around 0·5Hz, coinciding with track inputs of 110m at 200 km/h, or 140m at 250 km/h. The immediate solution to this is application of improved control systems to track maintenance machines, such as automatic track top and alignment equipment. More interesting solutions currently being developed include semi-active or active suspension using simple control strategies such as skyhook damping (that is, damping relative to an absolute datum).
In the longer term, more exciting technical possibilities exist, using the science of mechatronics - the integration of electronic or IT systems with mechanical engineering. The train could be capable of learning the track quality, and a sophisticated active suspension control used to overcome geometric irregularities. Such a train could maintain comfort on relatively imperfect track, and with the cost of track maintenance a major factor in operating any railway, the attractions of this technology are clear. Suspensions in their conventional form could ultimately disappear altogether, and the initial development of such trains is already under way in Japan, North America and Europe.
Braking within existing signalling distances presents one of the greatest problems in operating at high speed on conventional lines. Unless heavy and relatively expensive magnetic rail brakes are employed, the demands on energy absorption capability and on adhesion are considerably increased.
Development of the 200 km/h diesel High Speed Train in Great Britain involved an increase in the braking rate from 7% to 9%g. Further speed increases of 10% will each require around 2% extra adhesion.
The capacity to absorb energy from higher speed in existing stopping distances is a critically limiting factor. Energy transfer rates are proportional to the train speed cubed, hence the use on high-speed trains of four sets of discs per axle, or eddy current brakes, with consequent escalation in the cost of both train and track maintenance. An obvious approach is a concerted effort to reduce train mass, and regenerating traction systems provide an alternative energy link capable of absorbing energy in proportion to the number of axles motored.
Insufficient adhesion is a rare but significant occurrence and will always become a more challenging issue as higher braking rates are demanded. Moreover, available adhesion decreases with speed, exacerbating the problem. Fortunately, two modern developments have significantly reduced the scale of this problem. Firstly, wheelslide protection equipment has been developed to maximise use of available adhesion by operating at a defined level of slip, helping to clean the railhead. In addition, controlled application of sand in braking is now becoming accepted.
These two techniques provide the opportunity for some increase in speed without lengthening signalling distances. Although the limit of passenger acceptability will be reached eventually, it should be possible to operate with some increase in speeds on conventional lines without recourse to the use of magnetic rail brakes.
Operation at higher speed on conventional lines poses many challenges in the field of aerodynamics, including the effect of pressure pulses in tunnels and in the open air, and slipstream effects as trains run through stations or pass trackside workers.
Pressure pulses in tunnels probably present the most extreme difficulty. The pressure waves generated when a train enters a tunnel travel at the speed of sound along the bore and are reflected back from the exit portal, giving rise to significant changes in pressure through which the train must then pass. In single or double-bore tunnels this can require a speed limit to ensure the aural comfort of passengers. The development of the pressure waves in a tunnel as predicted by the Thermotun program is shown in Fig 1 (above right).
This tunnel problem can be minimised with air shafts to relieve pressure or flared tunnel entrances, although each obviously has practical limitations. Pressure-sealing the train is expensive and gives rise to long-term maintenance costs.
In the open air, pressure pulses are generated by any passing train, giving rise to the startle effect on passengers and transient structural loads on the sides of trains. Increasing train speeds and close track spacings exacerbate the problem. This can be reduced at the design stage with an elongated train nose, but little can be achieved retrospectively without increasing the track spacing.
Safety on platforms is also perceived to be an issue, because the slipstream generated by trains could have the effect of unbalancing waiting passengers and moving their belongings along the platform. This effect has been well understood for many years, and is the origin of the yellow lines painted on platforms through which high speed trains pass. It is, however, interesting to note that the effect of irregularly-shaped freight trains at current speeds can be at least as disruptive as very high speed trains.
Finally, in extremely high cross-winds, there is the possibility that a lightweight train might overturn. The risk increases with speed, and careful regulation of speed in high winds may be necessary, as is already the case in Japan.
Noise and vibration
With higher speeds, levels of noise and vibration also tend to rise, and with current designs a 50% increase in speed will increase maximum noise levels by over 5dB. An initial method of control is provided by reducing the surface roughness of wheels and rail, through periodic rail grinding and the avoidance of wheel damage through braking for example. Further reduction in rolling noise would require the use of noise barriers. At very high speeds, aerodynamic noise becomes dominant, but as this does not occur until well over 300 km/h, it is not a problem for conventional lines.
Higher speeds on a given quality of track could increase ground-borne vibration, but in practice the need to control the forces going into the track for other reasons tends to resolve this problem. There is often a problem of ground-borne noise from tunnels, resulting from vibrations passing through the ground and causing the surfaces of nearby buildings to radiate a rumbling sound. Higher speeds in tunnels will mean greater levels of ground-borne noise, unless the isolation of the track from the ground is increased in the appropriate frequency regime.
Current collection and supply
The fundamental dynamics of overhead current collection have been understood since the mid-1980s. In principle, higher speeds can be achieved on any design of overhead equipment by use of a pantograph with low mass, low head suspension stiffness, and neutral aerodynamic characteristics. Understanding of these principles by BR Research led directly to the development of the Brecknell Willis high-speed pantograph and allowed a speed increase of at least 25%, which was a significant factor in the economic case for electrification of Britain’s East Coast main line between London and Edinburgh. The same approach was subsequently adopted on the Northeast Corridor in the USA.
Although operating today at 200 km/h, the ECML catenary and pantograph system is capable of operation up to 225 km/h and beyond, provided some attention is given to particular features of design, installation and maintenance, especially at level crossings.
Energy consumption and power demand, critical in terms of infrastructure provision and environmental impact, are proportional to the speed squared and cubed respectively, with an increase of 10% in train speed raising aerodynamic drag by 20% and power demand by 30%. However, modern engineering improvements can offset this.
Developments such as the Bombardier B5000 bogie planned for the Virgin CrossCountry fleet will create double benefit. The inside-framed bogie configuration will reduce aerodynamic drag while its lower mass will reduce tractive force demand for a given acceleration.
Similarly, lightweight three-phase induction motors and hopefully lighter traction converters will reduce power demand, particularly when starting. Regenerative braking will also help. In future, train designers should be able to achieve high speed without increasing current demand, thereby avoiding expensive alterations to the power supply.
Signalling and line capacity
Increasing speed raises a number of issues for signal engineers, some of which are entirely technical, while others are based on historic safety decisions. If braking within existing distances is unachievable, line capacity is seriously threatened, which may require a general reduction, or the complete prohibition, of existing traffic.
Some quite significant revisions of thinking will be needed, including better overall traffic regulation, development of ’flights’ of trains in the timetable, or ultimately additional infrastructure to reduce the impact of capacity loss. Use of sophisticated capacity modelling will help identify the optimum solution.
Longer braking distances require signalling alterations. With conventional signalling this can be achieved by increasing the length of the block sections, or more economically, the section length can be retained and additional aspects provided.
The use of additional signal aspects involves human factors as well as technical considerations. There is an upper limit to the train speed at which, under normal conditions, visual sighting of lineside signals is acceptable; in Great Britain this speed is taken to be 200 km/h. At higher speeds it is necessary to consider some form of cab signalling, whether an additional aspect in the cab, or some form of display which continuously indicates the maximum permitted speed.
Related to the cab signalling issue is the question of the warning and train protection systems. In Great Britain, the Railway Inspectorate has ruled that nothing less than cab signalling and full automatic train protection is acceptable for speeds in excess of 200 km/h.
The more radical approach to signalling for higher speeds is to use a form of transmission-based train control, such as that being developed for Britain’s West Coast main line. Inherently, TBTC provides cab signalling and full ATP functionality, and also the potential for moving block operation which helps alleviate capacity problems.
Dynamic forces between wheel and rail increase considerably at high speed, particularly at rail welds, insulated joints and switches and crossings where the impact forces are significant. In Britain, a yardstick was first provided many years ago with the stipulation that new trains should develop forces no greater than a Class 55 diesel-electric locomotive at 160 km/h on a defined dip, which allowed a trade-off between axleload and unsprung mass. The developing European standard concentrates more on lower axleloads.
The key consideration is the potentially increased deterioration of track, with a corresponding increase in the maintenance requirement and life cycle costs. Fortunately, this is not all bad news, and those factors which increase passenger comfort are also those which reduce the maintenance requirement. Thus, the key to higher speeds, from a track point of view, is to apply the fundamental principles of track behaviour developed in the 1970s.
In the early days of tilting train operation, much concern was expressed over track shifting and overturning. It was shown that these concerns were unfounded at intended speeds of operation, although of course, the ultimate overturning limit is approached more closely, so overspeeding must be properly addressed.
Other safety concerns
Two general issues have not been covered here: the integrity of key systems and the effect of external disturbances such as vandalism. The integrity of key systems such as axles has always been vital and fortunately, many can now be monitored. Bogie vibration is monitored on the TGV, and if hunting is detected a speed reduction is imposed. Similar systems are mandated in the US Federal Railroad Administration safety specification.
Other parameters such as out-of-round wheels or declining brake performance can be monitored, and modern systems permit predictive assessment, detecting faults before they become a safety issue and require speed restrictions to be imposed.
Vandalism is, in many ways, a societal problem, which the railway must deal with as realistically as possible. Questions of how a train designer must go about engineering solutions to problems posed by delinquent teenagers can be argued at length, and policies are quite different across Europe. In Britain, crashworthiness requirements include distributed energy absorption and designs of obstacle deflectors which are already more stringent than elsewhere in Europe, while French National Railways has concentrated on achieving a high degree of energy absorption at the very front of the train. Considerable scope exists for an approach which takes into account the legitimate fears of the general public, but at a price which does not drive railway customers onto the roads forever.
Systems engineering challenge
There is a significant variety of interacting technical problems. Much background understanding exists, and given sufficient money and time, solutions to all of these problems appear possible. However, money and time are at a premium for railways worldwide, and fresh thinking is called for.
Hitherto, the tendency has been to consider the problems as a set of disconnected technical issues. The modern development of systems engineering to consider issues holistically rather than in a piecemeal manner promises to be a far better way forward. By this means a whole problem can be stated, such as the maximum speed obtainable within a given investment budget, and the railway engineering community challenged to deliver a coherent, cost-effective and complete solution. o
CAPTION: Fig 1. The development of the pressure waves generated by a train running at 225 km/h in a 3 115 m long tunnel, as predicted by the Thermotun program
CAPTION: Bombardier’s B5000 bogie is currently under development for the Virgin CrossCountry fleet
Grande vitesse sur lignes existantes
Atteindre des grandes vitesses sur des lignes conventionnelles offre des perspectives commerciales à des coûts marginaux, comparés aux coûts de construction d’infrastructures nouvelles. Le défi technique global que représente la circulation des trains à 225 ou 250 km/h sur lignes existantes est plus difficile à surmonter que la construction de lignes nouvelles conçues pour cela. Considérant ce challenge dans son ensemble, chaque point d’ordre technique doit être identifié, compris et surmonté afin de constituer un tout qui soit efficace et fiable. Ces points englobent le confort, le freinage et les effets de compression dans les tunnels et dans les traversées de gares, aussi bien que la signalisation et le captage de courantHochgeschwindigkeit auf konventionellen Strecken
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