Operating at High Cant Deficiency (continued) High-Speed Curving Issues
For passenger trains, the primary high-speed curving issue is lateral acceleration. Rail passengers expect to safely and comfortably stand up and walk around during their journey. Standing passengers are thus the worst case. Accelerations on the order of 3-4 ft/sec² are generally the maximum acceptable.
Passenger comfort in curves can be improved by tilting the vehicle body. This reduces the perceived lateral acceleration, thus allowing higher curve speeds. The first tilting train was the Advanced Passenger Train (APT) in the U.K. This design was unsuccessful, principally due to project management, rather than technology. Successful designs, coming 10 years later, were the X2000 in Sweden and the Pendolino in Italy. The Pendolino is a clear commercial success with trains delivered to nearly a dozen countries. Tilt trains of various designs are now used in almost 20 countries. The Acela trainsets are a domestic example.
Figure 3 shows a Pendolino train running through an S curve (note that each car tilts independently). The tilt mechanism is a pair of hydraulic cylinders and inclined links on each truck. These act to rotate the car about a point 60 to 70 inches above top of rail. This corresponds roughly to passenger head or chest level. Experience has shown this roll center maximizes perceived comfort. Tilt trains typically run at maximum uncompensated accelerations of 5-6 ft/sec² with 60% to 70% compensation. Full compensation leads to greater discomfort. Passengers want some perception of curving, otherwise visual and inner ear signals conflict, leading to motion sickness.
Safety-related issues must also be addressed. What is the derailment risk at high cant deficiency? Are the lateral forces guiding the vehicle sufficient to cause it to leave the track, perhaps even overturn? Inversely, are the lateral forces large enough to cause track shift? Some of these issues will be illustrated by modeling results for a vehicle patterned after the newest generation Pendolino. The calculations were done using the VAMPIRE® vehicle dynamics model.
Figure 4 shows steady-state curving forces at balance speed in a 750-meter curve (nearly 2-1/2 degrees). The arrows depicting lateral forces are normal to the arc representing the curve. The arrows depicting longitudinal forces are tangential. The lateral forces are the force of the rail on the wheel. For both the outside and inside curve rails these forces are directed towards the track center. This means the wheels are attempting to spread the gauge and the rails are resisting. Gauge-spreading forces are large at the leading axle and minor at the trailing axle. The axle steering moment is large at the leading axle and relatively small and in the opposite direction at the trailing axle. The leading and trailing truck patterns are similar.
AUGUST 2004 "Effects of Rail Cant on Wheel/Rail Forces and Derailment Potential"READ ARTICLE
DECEMBER 2004 "Flange Climb and Independently Rotating Wheels" READ ARTICLE
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Passenger comfort in curves can be improved by tilting the vehicle body. This reduces the perceived lateral acceleration, thus allowing higher curve speeds. The first tilting train was the Advanced Passenger Train (APT) in the U.K. This design was unsuccessful, principally due to project management, rather than technology. Successful designs, coming 10 years later, were the X2000 in Sweden and the Pendolino in Italy. The Pendolino is a clear commercial success with trains delivered to nearly a dozen countries. Tilt trains of various designs are now used in almost 20 countries. The Acela trainsets are a domestic example.
Figure 4 shows steady-state curving forces at balance speed in a 750-meter curve (nearly 2-1/2 degrees). The arrows depicting lateral forces are normal to the arc representing the curve. The arrows depicting longitudinal forces are tangential. The lateral forces are the force of the rail on the wheel. For both the outside and inside curve rails these forces are directed towards the track center. This means the wheels are attempting to spread the gauge and the rails are resisting. Gauge-spreading forces are large at the leading axle and minor at the trailing axle. The axle steering moment is large at the leading axle and relatively small and in the opposite direction at the trailing axle. The leading and trailing truck patterns are similar. 
