Distributed-power, 200-mph pantographs, and really wide turns are some of the features of today’s high-speed train travel.
High-speed trains crisscross Japan, much of Europe, and are starting to fan out across China. Here’s a look at some of the technological advances made in high-speed travel by two major train manufacturers, Bombardier in Canada, and Siemens, a German firm.
Train sets and locomotives
State-of-the-art high-speed (HS) trains don’t follow the traditional rules of the rails. Older trains still use one or two locomotives that contain all the traction motors and locomotives are positioned in the front as a pair, or the front and rear to better distribute the tractive forces.
Modern HS trains, however, put traction motors in almost every car for better force distribution and a more-comfortable ride. Distributing power over more axles also lets trains accelerate and decelerate faster. That’s because the powered wheels rely on friction between the wheels and rail to transmit power. Sending all the power to only two or three axles, especially when starting out from a dead stop, increases the likelihood that the power would overcome the friction and spin the wheels. There are also conditions, even with distributed power, when there’s not enough friction to get a large train rolling or up an incline. Ice or wet leaves, for example, can severely limit the frictional force and cause wheels to slip. To overcome this, HS trains carry dry sand, just like their slower-speed cousins. It gets dropped in front of powered wheels to increase frictional forces.
Spreading the tractive force between axles, mounting the traction motors below the floors, and eliminating the locomotive can give modern HS trains 20% more space for passengers in the same-length trains, according to Siemens.
The train set, which includes all the passenger cars, has a driving car. It is the lead car and features a sloped, aerodynamic nose and a relatively small compartment at the front of it to accommodate a windshield, the controls, and communication equipment, as well as the driver, the only person really needed to drive the train. The rest of the driving car is outfitted for passengers.
Most train sets are made up of eight to 10 cars hooked together with a driving car in the lead. But both Bombardier and Siemens make cars that can be part of 16-car train sets, which are about 1,400-ft long, or over a quarter-mile. The weight of such a train, including 1,200 passengers and their luggage, is around 1,000 tons.
On Bombardier’s Zefiro, configured as a 16-car train set, there are normally 32 traction motors, so half of the axles are powered. (Each car travels on a pair of two-axle bogies). And each motor contributes 600 kW, or 800 hp, giving the entire train set over 19 MW of installed power. Energy consumption, which varies with operating conditions, averages 0.08 kW-hr/mile/seat.
Siemens sums up energy efficiency another way, saying their Velaro train sets consume the equivalent of 0.33 liters of gasoline (about the amount of liquid in a can of soda) per seat per 100 km. And in terms of the environment, they emit about 14 gm of CO2/passenger/mile.
To cut energy use, Bombardier’s HS trains rely on permanent-magnet motors, which need 4% less energy than asynchronous motors.
When HS trains are just getting rolling, they accelerate at about 1.6 fps2, which eventually increases to about 3 fps2, the upper limit in terms of passenger comfort. Braking deceleration is also about 3 fps2. This means a train traveling 200 mph takes about 2.4 miles to come to a stop, according to Siemens.
Power and braking
Most HS trains get electricity from overhead wires or catenaries using a pantograph. Today’s batteries could never be sized to supply the power needed and still fit on the train. Diesel engines turning generators is not considered environmentally friendly and the weight and storage of diesel fuel, along with fire safety, would pose other problems. Another option, using a shoe to take electricity from a third rail, much like light rail, creates too much friction between the shoe and rail at high speeds.
The biggest challenge with using pantographs to take power from the catenary is keeping the contact forces between the two within a given range — not too much friction but enough contact to make a solid electrical connection. And, according to Siemens, the technology for catenary/pantograph subsystems has been under development for decades and can be considered mature.
Another issue that crops up with catenaries is when the right weather conditions result in ice on the overhead wires. In that case, the train deploys two pantographs (there are often several on a train set) and the lead one knocks off the ice. This maneuver is not done at maximum speed, says Siemens. To handle travel in either direction, trainmakers often package a pair of pantographs in the same overhead fairing, mounting them face to face.
Like most electric cars, HS trains feature regenerative braking, that is, using the traction motors as brakes, which generates electricity. And usually this electricity is fed back through the overhead lines so it can power an accelerating train on another track or get stored for a short time before being used by another train. There’s not enough room to store recouped electricity efficiently onboard the train and it rarely gets fed back into the national grid. When there is no place to store or use the electricity, it gets burned off in roof-mounted braking resistors (rheostatic braking) or the train switches to friction braking.
While regenerative braking is preferred because it minimizes wear and tear and saves energy, trains also carry electropneumatic friction brakes outfitted with steel discs and sintered pads designed for high loads.
Siemens’ Velaro train features linear eddy-current brakes. They consist of electrical coils positioned along the rails and held about 7 mm above them. The coils serve as magnets, but their north and south poles are continually switched. When the magnets move along the rail, their changing magnetic field creates another field in the metal rail, which creates electrical tension and eddy currents. The interaction between the coils’ magnetic fields and eddy currents they create in the wheels provides enough resistance to slow the train. Eddy-current brakes work fine at high speeds but are not viable at low speeds.
HS trains have two levels of emergency brakes. The first uses all forms of brakes onboard. In the second, called safety brakes, there is no electricity available, so only friction braking comes into play.
Aerodynamics and efficiency
Train designers are always striving to make their trains more aerodynamic. That’s because up to 60% of the tractive force can be lost due to drag and friction. Aerodynamics must also make the train stable because as speeds increase, cross-wind stability decreases.
Bombardier engineers use CAD and CFD to model as many as 60 design parameters. They concentrate on four basic areas: the lead car with its streamlined nose; spoilers atop and beneath the cars; stowing and deploying the pantograph; and fairings around the bogies, between cars, and covering the underside of the train. Fairings around the bogies also prevent track ballast and debris from damaging the wheels and axles. Through repeated efforts, Bombardier cut drag by 25%, which yielded a 15% drop in energy use and reduced the wind force on the lead car by enough to let them get rid of the 5 to 7 tons of dead weight the trains carried for ballast and stability.
Covering roof-mounted equipment with streamlined fairings also cuts drag, as well as limiting the “sonic booms” the exposed equipment can cause when trains enter a tunnel. The smoother roof cuts overall outside noise, too. For example, when passing another train at 186 mph, Siemens’ Velaro produces 91 dB(A) of noise. On the inside, passengers are exposed to only about 72 dB(A), thanks to sound-dampening coatings on the inside skin of the car and sound dampening on the air conditioner’s inlet ducts. There are also noise absorbers on the wheels, as well as air-spring bellows between the bogie and car body.
Another area train designers concentrate on is HVAC. Trains running in extreme temperatures, hot or cold, devote up to 30% of the total electricity used on HVAC. To get the most out of their HVAC units, Bombardier varies the air intake based on passenger occupancy, thus minimizing heating and cooling needs. They also modified the air conditioner so that it works as an efficient heat pump in cold climes. And before venting cabin air outside to keep air inside the cabin fresh, it is routed through heat exchangers to preheat or precool incoming air. This lets the HVAC equipment reuse up to 80% of the energy it spent cooling or heating the cabin air. Overall, Bombardier efforts led to a 25% drop in energy used annually on HVAC.
One of the features of Europe’s HS railway that make it so convenient is that many of them seem to effortlessly cross borders. But behind the scenes, engineers and designers have worked hard to make this possible. Bombardier’s AVE S-130 train in Spain, for example, can be powered by 3-kVdc or 25-kVac power, thanks to two sets of transformers, converters, pantographs, and other electrical gear. The train’s variable-gauge bogies let it travel on rails 1,435 mm apart (standard gauge), as well as those 1,668 mm apart (Iberian gauge). And the Zefiro train, among others, can be configured for up to four different power grids: 1.5 and 3 kVdc and 15 and 25 kVac.
This lets trains move easily between different gauge tracks and different electrical grids.
Trains also must be able to use the different safety signals and meet track height, width, and weight limits. Engineers have yet to make trains that can change dimension, so they design to the strictest size limitations the train will likely see. But they have made versions that can switch between safety and signaling standards. Currently, train manufacturers must juggle six different national train-control systems, as well as the European Train Control System. (ETCS standardizes signaling, control, and train protection. It was supposed to replace various incompatible safety systems used by European railways, especially on high-speed lines.) A European group is now working to develop a standardized train-control system for all participating countries.
What constitutes high speed
Bombardier pegs high speed as at least 118 mph, which means they don’t necessarily need special tracks. Trains that go 186 mph and faster are called Very High Speed (VHS) and do need upgraded tracks.
Tracks for high-speed trains
Another limitation is that HS trains cannot negotiate tight turns and keep passengers comfortable. So trains running at 125 mph and faster usually travel on tracks with curve radii of 2.8 miles or greater. HS trains topping out at 217 mph run on tracks with curve radii of 4.3 miles or greater. On HS turns, the outer rail is often 150 mm higher than the inner rail to create a 1:10 banking angle, according to Bombardier. Proper banking makes for a more-comfortable ride.
HS trains are also limited in terms of the grades they can easily handle. In general, grades must be 3.5% or less. Any higher and the train slows down and becomes inefficient.
The cost of building a two-track high-speed rail line, one that will let trains travel at top speed in both directions simultaneously, has been estimated by the U. S. Government Accountability Office at $50 million/mile. This can vary depending on the train, whether the rails will run through urban areas, underground, or empty countryside, and how many other roads and rails need to cross over or under the HS rails. There are no at-grade crossings on HS rails carrying trains at over 125 mph, a universally accepted safety measure. For HS trains traveling at 110 to 125 mph, FRA permits crossings in the U. S. only if an “impenetrable barrier” blocks highway traffic when a train approaches. Another cost in building HS rails are the overhead lines from which trains draw power.
Another cost variable is whether the designers use ballasted track, which are traditional tracks with cross ties kept in place by gravel (ballast), or slab tracks. Slab tracks mount the rails on concrete track rather than using ballast. Slab track costs more to build but is less expensive to maintain.
Estimates for annual maintenance costs on HS rails are not clear. A European study pegs maintenance costs at $140,000/yr/mile, while a British study of HS rail put the annual cost at $493,000/mile. Maintenance is so high because as speeds increase, tracks need to be inspected more often and to tighter alignment tolerances.