Brakes

Current TGV trainsets use three brake systems: disks, dynamic brakes on powered axles, and in some cases tread brakes for emergencies. For speeds of 350 km/h (218 mph) and more, these conventional brakes lose their ability to stop a train in a reasonable distance and cannot perform a safe emergency stop. The basic problem is simple: kinetic energy, which must be dissipated as heat in the braking system, grows as the square of speed. Braking is therefore a major technological hurdle, which must be overcome to safely attain higher speeds in revenue service.

Traction Equipment

Traction for the TGV NG is to come from asychronous 3-phase AC induction motors. This choice is motivated by simplicity of maintenance, very high power to weight ratio (around 1 kW/kg), and higher RPM limit. The 2 x 6000 kW traction package, 40% more powerful than 2nd generation units, requires that a total of 6 powered trucks be used instead of the usual 4, so that sufficient tractive effort can be put to the rail despite the low (17 metric tons) axle load. The extra two powered trucks are located under the first and last trailer, immediately adjacent to the power cars, as used for the TGV Sud-Est or Eurostar. The trainset is designed to be capable of starting on a 4% (40 in 1000) grade with two traction motors shut down. Cooling of the traction equipment must remain effective at ambient temperatures up to 45 degrees C (113 degrees F), using an environmentally friendly liquid coolant (FC72 by 3M) for the semiconductors rather than freon. Traction motors are individually controlled, whereas they were previously controlled in pairs using one inverter. Reactive currents generated by a trainset must be under 1 amp, to avoid perturbing the cab signal and communications channels in the rails; this is in comparison to the 15 amps of a TGV Atlantique trainset. Overall, there is nothing radically new in the traction equipment; the big challenge lies with satisfying a very large power and tractive effort requirement, within drastic axle load constraints.

Cutting Down Weight

Axle loads become a critical constraint at high speeds, in order for maintenance costs of track and train to be reasonable. The current axle load limit is 17 metric tons, and there is a possibility that this will be reduced to 16 tons for the faster TGV NG. Meanwhile, it is desirable to retain an articulated design (with two axles per trailer), a bilevel seating arrangement, and a host of other requirements that would tend to increase the axle load of a trainset. It turns out that keeping weight down is one of the biggest challenges for the design of the TGV NG or a potential tilting TGV.

The first approach to reducing weight is new materials. For the TGV Duplex car bodies, aluminum is used. The trailers are a monocoque design assembled out of extrusions, yielding a weight reduction of 20% over an equivalent steel structure. The frame of the power cars is made of high tensile strength steel, as in the TGV Atlantique units, for a weight reduction of 10% over lower grade steel. Stainless steel could also make an entry into the new trainsets, as well as composites. Composite materials are not used on the TGV Duplex's main structural components for reasons of cost, and also because the technology was not deemed sufficiently mature. Future TGV generations, however, could be built with a composite main structure assembled with glue. There is a research effort to explore the resistance of composite materials to the wear and tear encountered over 30 years of high-speed operation. Other weight reductions are achieved by using better paints, electrical wires with thinner insulation, and many other small measures that become significant when added together.

The second way to cut weight is by using the least possible material to fulfill structural requirements, or optimization. This has become a worthwile pursuit with the advent of extensive computer finite-element analysis. The TGV Duplex is the first to really benifit from these techniques, and the TGV NG will follow suit. Several areas yield weight improvements. The connection between trailers has been completely redesigned, and is now attached to the trailer bodies rather than the truck and suspension assembly. This allows a substantial reduction in the weight of the secondary suspension, with a 400 kg (880 lb) savings. The Y237B truck used on today's TGV Atlantique has been redesigned, saving 200 kg (440 lb). An aluminum version was tried, but is not ready to be integrated into the TGV NG. The interior, as with commercial transport airplanes, is designed to be feather weight. Seats have been entirely redesigned for the TGV Duplex, with each one going from 26 kg (57 lb) to 14 kg (31 lb), yielding a full 1000 kg (2200 lb) reduction for each trailer.

Noise

Crashworthiness

Following recent trends in the auto industry, there is significant effort going into passive security of TGV trainsets. As it stands, the TGV architecture already behaves quite well in accidents, as demonstrated in the December 1993 high speed derailment of a TGV Réseau trainset, and various other encounters with large, intruding objects on the track. A particularly pressing motivation for this research was the aftermath of a crash on 23 September 1988, when TGV Sud-Est trainset 70 rammed into a low-boy trailer stuck on a grade crossing, at a speed of 105 km/h (65 mph). The train did not derail, but the engineer died in the incident. It was found that about 10% of the crash energy had been absorbed in deformations of the first trailer; at higher speeds, serious injury to passengers in this first trailer might have resulted.

Using a new software for railroad crash simulation, called 'Pamcrash', the TGV Duplex structure was optimized on a supercomputer. The trailers have extremely rigid bodies with deformable, energy absorbing crush zones at the ends. The coupling between the power cars and trailers has also been reviewed; it still uses screw-link couplers with buffers, but the buffers have structural fuses built in so that they fold away under crash loads. The car ends have been reviewed to prevent the power car from climbing onto the first trailer. For multiple unit operation, the Scharfenberg couplers in the noses of the trainset have been designed to collapse under heavy loads, so that two power cars coupled nose to nose can make firm contact with each other to prevent telescoping. This is in addition to the energy absorbing ram shield already mounted in the nose of all TGV power cars to defend the cab cubicle.

To determine the effectiveness of these design changes, full size crash tests are conducted with specially instrumented production vehicle shells. For the TGV Duplex program both an end-trailer and a power car were rammed into a barrier at 40 km/h (25 mph), validating most of the computer model predictions.

To Tilt, or Not To Tilt...

With ongoing efforts by the French government to bring costs under control, the policy of heavy investment in new high speed lines is losing momentum. With already three major trunk lines radiating out of Paris, a potentially cheaper way to extend the benifits of high speed rail is to improve the performance of TGV trains when they travel on standard trackage.

One possible way to achieve this is through tilting the trailers to permit higher speeds in curves without causing discomfort to the passengers. It should be noted that this feature provides no advantages on dedicated high speed lines, where curve radii and superelevation are deliberately designed for high speed operation. But even the advantages on standard trackage are small. The cant deficiency permissible on French tracks (considerably higher than in other European countries) already allows all trains (including TGVs) to practice relatively high speeds in curves. The addition of a tilt system does not change the lateral loads applied to the rail in curves, so the major obstacle to higher speeds becomes the axle loading. This will probably prove too difficult to overcome for high speeds without entirely changing the train's architecture to distribute traction along its entire length, such as done in the Italian ETR 460 Pendolino or the German ICE 3 designs. The only definite advantage of a tilt system would then be better comfort for passengers, and it is not clear that this would justify the investment.

Short TGVs

It has been found desirable to increase the operational flexibility of TGV trainsets by making them shorter, with only four trailers instead of eight or ten or more. After running coupled together on a high speed line, the trainsets can separate and continue to different destinations. This can extend the benifits of TGV service to places with insufficient traffic to justify service with full-size trainsets.

The idea is to adapt the TGV Duplex design, yielding a 235 seat trainset with one power car and four trailers. The new element in this design is an articulated driving trailer, which would fulfill the dual function of bar trailer and driving cab. Its nose would be indistinguishable from that of the power car and would contain similar crash protection. The major obstacle to this design is the presence of passengers in the first vehicle of the train, which could compromise safety in the event of a collision. This concern was mitigated with recent advances in rail vehicle crashworthiness, as well as the equivalent German practice.

Last Modified: November 2000