With trains operational in more than a dozen countries and two more fleets about to enter service in Britain, the commercial case for tilt is firmly established. While there is still diversity of actuation techniques, the cost is falling and side effects such as nausea are being overcome

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Roger Goodall is Professor of Control Systems Engineering at Loughborough University. Stuart Brown is Senior Consultant at Halcrow

TILTING TRAINS began to emerge in the 1970s, but early attempts were not very successful. The first UAC Turbotrain entered service in the US in 1972 but made no impact. Although a later version ran for a time between Montréal and Toronto, this too proved unreliable. Britain's Advanced Passenger Train lasted less than a week in public service. The cardinal error in both cases was an attempt to concentrate too much innovation on one train.

The first tilting fleet to stay the course was the six-car Series 381 EMU, introduced in 1973 between Nagoya and Nagano in Japan. Passive tilt up to 6í was achieved by allowing the body to rotate on rollers mounted on top of the secondary air springs. Problems with nausea soon emerged, and plans to raise the maximum speed on this mountainous 1067mm gauge route from 120 to 130 km/h were postponed.

But this was a regional service - serious inter-city business in Japan was being handled by the expanding Shinkansen network. Elsewhere in the world, the aim was to upgrade prestigious high-speed routes without building a new line. This produced the Talgo Pendular in Spain (1980), the LRC as successor to the Turbotrain in Canada (1982), the first ETR450 Pendolino trains in Italy (1988), and the X2000 in Sweden (1990).

The 1990s have seen tilt mature into a standard railway technology, with applications extending to more than a dozen countries including 10 in Europe. All the major manufacturers now offer tilting trains both for regional and high-speed applications.

What tilt achieves

Since the 1970s, there have been substantial changes in our understanding both of the applicability of tilt and the technology available to achieve it.

Tilting trains exploit the fact that speed through curves is principally limited by passenger comfort, and not by either lateral forces on the track or the risk of overturning. The principles and basic equations related to tilting are well-known1, but it is useful to review them in relation to operational advantages.

Two primary decisions need to be made. The first is the maximum tilt angle to be provided (qtilt); this is based upon the mechanical design of the vehicle, especially taking gauging issues into account. The second decision is what cant deficiency the passengers should experience on a constant radius curve (qCD tilt), which is of primary importance to comfort.

Given these two decisions, and the value of cant deficiency that applies for the non-tilting case (qCD non-tilt), it is possible to derive an equation for the increase in curving speed, or speed-up, offered by tilt:

Maximum track cant is usually 6í, and typically 6í of cant deficiency is specified for a non-tilting train. Applying 9í of tilt and with a cant deficiency of 6í for the tilting train, the calculation indicates a speed-up of 32%. Another example might be where the tilting cant deficiency is reduced to 4·5í, perhaps to offer an improved performance; using a slightly smaller tilt angle of 8í, the speed-up falls to 24%.

Steady-state curving speed increases of around 30% are often quoted for tilting trains, but the comfort level on transition curves is actually more critical, though less frequently discussed. Seated, standing and walking passengers easily adapt to uncompensated lateral acceleration in a long circular curve. In a transition curve, and particularly reverse curves, standing or walking passengers - and waiters pouring hot coffee - have to react very quickly by leaning into the approaching curve, much as a cyclist does, or they will fall over.

Seated passengers are pushed over into the right position by contact with their seat when the coach tilts into a curve. Anyone walking along a coach is effectively pin-jointed to the floor through one ankle. Tilting the coach provides visual clues, but no actual force pushes their body into a leaning attitude. So the absolute lateral jerk rate through the transition has to be compensated by swift muscular reaction.

Curve transition comfort can be predicted on the basis of three things: the lateral acceleration, the lateral jerk rate, and most importantly the body roll velocity2. Uncompensated lateral acceleration during the transition will rise to the steady curve level, usually the same or a little lower than the non-tilting case, but jerk will be higher because the train is travelling faster.

For the roll velocity there is a trade-off here: we know that increasing the amount of tilt reduces the curving acceleration, but at the same time roll velocity is increased. For the 9í tilt, 6ícant deficiency case mentioned earlier, it can be shown that there is more than a threefold increase compared with the non-tilting case. How significant this is depends upon the transition time.

For a typical transition lasting just over 3sec on a non-tilting coach, the transition comfort level for standing passengers is significantly deteriorated, and it is necessary to operate with only a 25% speed increase to ensure the same comfort level as the non-tilting train3. A minority of curve transitions are shorter than this, sometimes less than 2sec long, in which case further speed reductions are necessary.

There is also the issue of motion sickness, which is known to affect a minority of people travelling on tilting trains. In the early days, design engineers tried to reduce the steady-state curving acceleration to zero, but it was quickly found that this created a problem with motion sickness. This problem is substantially reduced by compensating for only a proportion of the curving acceleration. Two-thirds is a normal value used today.

In contrast to the curve transition comfort, which can be considered on a curve-by-curve basis, motion sickness is a cumulative effect that arises as a consequence of a number of human factors4, although the exact nature is not fully understood. Again the effect is aggravated on highly curved routes with short transitions. This is another factor affecting what proportion of the curving acceleration is compensated, and careful design and optimisation of the tilt control algorithms is needed.

In addition to reduced journey times that increase revenue, it is important to recognise that there are other economic benefits with tilting trains:

  • shorter round-trip times may reduce the fleet size;
  • line capacity may be increased by faster transit of signalling block sections;
  • brake pad wear will be reduced due to less frequent braking;
  • energy consumption will be reduced due to less frequent acceleration.

System design issues

The engineering and operating implications of tilting extend beyond rolling stock to track, structures, signalling and train control. Increasing speed on curves potentially increases lateral track forces, which requires assessment of the possible effects on the track, earthworks and structures, including inspection and maintenance work.

Curve transitions need to be maintained to a high standard to ensure a smooth response of the tilt system on the entry to curves. Where possible, it is desirable to lengthen transitions on certain curves to minimise the lateral jerk rate and the roll speed and acceleration.

It may also be necessary to reposition some lineside signals to allow increased stopping distance from the higher speed. Where tilting and non-tilting stock use the same line, there needs to be an indication to the driver of a tilting train of the enhanced speed permitted on any particular curve, which requires additional lineside signs, at least. Because the traditional margin of safety against derailing or overturning is eroded by higher speeds for tilting trains, it is sometimes considered necessary to introduce a speed control system.

Safety raises other challenges5. The coaches must remain within gauge under conditions of hard-over tilt failure in either direction. Where this is not possible, or if the tilt mechanism has failed, or the train has been diverted off a route fitted with curving speed control, the coach must be returned to the upright condition and stay there. This requires careful design and risk analysis.

Kinematic gauging requirements also mean that the coaches of a tilting train will be narrower and more rounded than a conventional train, which may lead to passengers feeling enclosed. Space for luggage racks above the seats may be reduced.

The potentially increased lateral forces acting on the outer wheels and rail in a curve must be reduced as far as possible. This can be done using improved bogie design, including steered axles6.

Another way of reducing curving forces is by minimising the overall mass, which has a further benefit in that it reduces the auxiliary power needed to tilt the bodies. However, mass reduction must be done carefully to ensure that the risk of vehicles overturning when exposed to high winds is controlled, a factor which also means that the vehicle designer must give careful consideration to aerodynamics. During high winds, it may be necessary to reduce tilting train speeds in exposed locations.

Tilt mechanisms

Broadly speaking, there are four mechanical arrangements which are possible to provide the tilting action.

The first is passive or pendular tilt, in which the longitudinal roll axis on the secondary suspension is raised to near roof level. The centre of gravity of the body is then substantially below the roll axis and the body naturally swings outwards, reducing the lateral curving acceleration experienced by passengers.

We have already noted that Japan's Series 381 EMUs achieved this by providing rollers above the air springs supporting a curved track fixed to the body. In the Talgo Pendular, the air springs are located on vertical pillars at the vehicle ends between the articulated bodies to achieve the same effect. But the response time of passive tilt is too slow to be particularly effective. Positive control involving significant power is needed.

A second approach is to apply active control to the secondary roll suspension. One method, which has been tried in both Europe and Japan, is to apply differential control to the air springs. Unfortunately, this causes a dramatic increase in air consumption and generally hasn't found favour, although there have been trials in Japan with a system transferring air between the bellows using a hydraulically-actuated pneumatic cylinder.

An alternative way is to use an active anti-roll bar (stabiliser). As applied in Bombardier's regional Talent trains, this uses the traditional arrangement consisting of a transverse torsion tube with vertical links to the body, except that one of the links is replaced by a hydraulic actuator and thereby applies tilt via the torsion tube.

A more common arrangement is a tilting bolster, the necessary rotation being achieved either by using a pair of inclined swing-links or a circular roller beam. An important distinction is where this bolster is fitted compared with the secondary suspension, and this makes the third arrangement.

Some vehicles, including Fiat's early Pendolino trains, have the tilting bolster above the secondary suspension, in which case the increased curving forces need to be dealt with by the secondary lateral suspension. Normally some form of centring method is needed to keep within the limits of travel, a pneumatic device in the case of the Pendolinos.

The final arrangement has the tilting bolster below the secondary suspension, thereby avoiding the increased curving forces on the lateral suspension. This is probably the most common of all systems. It was used in the APT, and is found in the X2000 in Sweden and the LRC in North America. The tilt-below-secondary arrangement is now being used in the Fiat-designed bogies for the Virgin West Coast Pendolino Britannico trains (p527).

It is clear therefore that during the development of tilt there has been a high degree of mechanical diversity, although there is now some convergence towards a common solution.

Control, sensing and actuation

Control of tilt actuation through curves requires complex processing of signals generated by sensors to achieve the desired dynamic response during the transition. Ideally, the tilt angle of the body should rise progressively, perfectly aligned both with the onset of curving acceleration and the rising cant angle.

If there is a delay, the increased curving accelerations are perceived briefly during the transition, even though they will fall to their design level on the steady curve. In principle, therefore, tilt action must be fast. But there is a complication because track irregularities have the effect of adding a random higher frequency element to the measured curving acceleration. Unless the tilt system avoids reacting to these irregularities, straight track ride quality will deteriorate.

Most tilt controllers are based upon lateral accelerometers fitted either to the body or bogie. These measure not only the curving acceleration but also the effects of track irregularities, and the output must be filtered to remove the latter effects.

Most modern tilt control systems use a 'precedence tilt' control strategy in which a tilt angle command is derived from a bogie-mounted accelerometer on the vehicle in front (Fig 1). This overcomes the transition delay that the filter would otherwise cause. Some tilt controllers use roll and/or yaw gyros on the bogie to facilitate the transition response, and there is currently strong interest in providing tilt commands from a track database synchronised to the movement of the train.

The most recent tilt trains in Japan such as JR Central's Series 383 use recorded route data fed into the tilt control mechanism. The route is selected by the driver at the start of a journey, and odometers on the train measure the distance travelled. A processor on board computes the amount of tilt needed when entering a curve that the train detects by the use of ground coils (RG 8.97 p532).

Actuators to provide tilt action are another area where there has been significant progress. Apart from the early systems based upon controlling the air springs (intrinsically pneumatic actuation), it was normal to use hydraulic actuators.

However, experiments with electromechanical actuators in Britain in the 1970s, and in Switzerland in the 1980s, paved the way for a progressive move away from the hydraulic solution. Electric motors controlled by solid-state power amplifiers drive screws fitted with high-efficiency ball or roller nuts to convert rotary into linear motion. Although they are less compact than hydraulic actuators at the point of application, overall they provide significant system benefits and are now used in the majority of new tilting trains.

Practical benefits

To assess journey time reduction over any particular route, it is necessary to carry out a simulation which takes into account the train and route characteristics, including the speed restriction profile allowed with the particular degree of tilt proposed.

A set of simulations applied to 160 km/h trains on main routes in Britain5, applying tilt but not otherwise altering performance, demonstrated these benefits:

  • Tilting trains provide significant benefits on West Coast and East Coast main lines as well as the CrossCountry routes operated by Virgin. Savings in the order of at least 6% are likely.
  • Smaller benefits can be realised on the Great Western main line and Trans-Pennine routes. Overall savings are in the order of 3% minimum, though much higher savings are possible on particularly curved sections.
  • The benefit of tilting trains increases with maximum speed because of the increasing negative impact of low speeds on curves.
  • Much of the benefit from tilting trains comes from increasing the maximum cant deficiency from 4·5í to around 8í. Increasing the maximum cant deficiency further has a diminishing benefit.

The benefits resulting from the introduction of tilting trains have been widely reported, and some of these are summarised below. It should be noted that these are not entirely due to the tilting ability as maximum speeds were increased at the same time:

  • The X2000 trains have cut the G?€?teborg - Stockholm journey by 22% from 225 to 175min.
  • German Railway's VT610 trains raised maximum speeds on the Nürnberg - Hof route from 130 to 160 km/h and cut journey times by 15% from 118 to 100min, although the trains have since experienced problems and have been withdrawn.
  • The Pendolino ETR450 reduced the Milano - Roma time by 20%.

Studies of the journey time savings resulting from tilt alone7 suggest that:

  • Talgo Pendular cuts 15% off journey times when running at a maximum speed of 160 km/h.
  • ETR450/460/470 in Italy has produced 12% reductions within the existing maximum speed.

A mature technology

Tilting trains have been in commercial service for almost three decades. Several hundred trainsets are now in operation in countries with widely differing conditions, including Spain, Italy, Portugal, Switzerland, Germany, Sweden, USA, Japan, Norway, Finland, Slovenia, Australia and France. Two major fleets are poised to enter service in Britain during 2002.

All of the large vehicle manufacturers have proven tilting train designs available. However, there is not as yet a standard solution for tilting. Many different variations exist. They include active or passive tilt, electric or hydraulic actuation, and control by acceleration measurement or route database and position detection.

Tilting trains are seldom used on new high speed lines and are by no means the universal choice for new high speed services on 19th Century lines or networks which do not have tight curves. Examples are the non-tilting Voyager for Virgin CrossCountry in Britain and three versions of the ICE in Germany.

Nonetheless, tilt is here to stay, and as it increasingly becomes a standard feature, the marginal cost of providing tilt should fall, especially as mechanical schemes and actuation technologies are developed. The cost of sensing and control is also falling.

There is a further spin-off from all the effort put into tilt development since the 1960s. Conventional rail vehicle suspensions were exclusively mechanical in nature, but now we have electronic designers and software engineers helping to determine suspension performance.

This represents a fundamental shift in design practice that will take the industry some time to absorb before it can fully exploit the benefits offered. Nonetheless, there is a reasonable probability that tilt is just the first stage in an active suspension revolution.

  • CAPTION: Swiss Federal Railways announced on June 26 that it will order 10 more seven-car ICN tilting sets from a Bombardier-led consortium. Including an option for a further 10, the contract will be worth €273m
  • CAPTION: Alstom is testing the first two Pendolino Britannico sets at 200 km/h
  • CAPTION: Queensland Rail recognises that passengers using its Brisbane - Rockhampton service may suffer from tilt nausea or 'motion sickness'
  • CAPTION: Fig 1: Precedence tilt control uses bogie-mounted accelerometers on the preceding vehicle to overome delays caused by filtering out the effects of track irregularities
  • CAPTION: German Railway's ICE-TD diesel tilting trainsets started running into Switzerland with effect from the June timetable change, following a test run to Chiasso (above) in May

References

1. Harris N R, Schmid F and Smith R A. 'Theory of tilting train behaviour', Proc IMechE, Vol 212, Part F, pp1-5, 1998.

2. Anon. 'Railway applications; ride comfort for passengers; measurement and evaluation', European Prestandard ENV 122999, Sept 1998.

3. Goodall R M, Zolotas A and Evans J. 'Assessment of the performance of tilt system controllers', Proc of the Railway Technology Conference, IMechE, Birmingham, November 2000, pp 231-239.

4. Forstberg J, Andersson E and Ledin T. 'Influence from lateral acceleration and roll motion on nausea: a simulator study on possible causes of nausea in tilting trains', Proc WCRR '99, Japan, 1999, paper CO3-4.

5. Brown S. 'An investigation of the options for new high speed trains in the UK', MSc(Eng) dissertation, University of Sheffield, August 1997.

6. Andersson E, Bahr H V and Nilstam N G. 'Allowing higher speeds on existing tracks - design considerations of the X2000 train for Swedish State Railways', Proc IMechE, Vol 209, pp93-104.

7. López Pita, A. 'Services offered by tilting body systems in relation to conventional vehicles', UIC-UNIFE-ERRI Interactive Seminar 'Can your railway benefit from tilting train technology?'

Tilt technology still evolving as the cost falls.

With trains operational in more than a dozen countries, and two more fleets poised to enter service in Britain, the commercial case for tilt on historic routes with significant curvature is firmly established. While there is still no consensus on the best actuation technique, the increasing number of trains ordered is bringing down the marginal cost of tilt. Sensing and control software development is improving ride quality and motion sickness is being overcome. Tilt may prove to be the first stage in an active suspension revolution

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Con trenes operativos en m? s de una docena de países, y con dos series m? s previstas para entrar en servicio en Gran Bretaña, se ha consolidado firmemente el argumento comercial en favor de los trenes basculantes en rutas históricas con curvaturas importantes. Aunque todavía no existe un consenso acerca de cu? l sea la mejor técnica de actuación, el cada vez mayor número de pedidos de trenes est? haciendo caer los costes marginales de la tecnología basculante. El desarrollo del software de control y de los sensores est? mejorando el confort del viajero y se est? n superando los problemas que daban lugar a mareos por el movimiento. La tecnología basculante puede demostrar ser la primera fase en una revolución de la suspensión activa