Interface - The Journal of Wheel/Rail Interaction

LOW-FLOOR VEHICLES

Introducing Low-Floor Vehicles into Older Transit Systems(continued)

This same sort of buckling can, and frequently does occur in the vertical direction. When a car is braking, the center of gravity is higher than the truck, so there is a moment that tends to push the back of the car up, and the front of the car down. The lead car body pulls up on the front of the center section and the trail one pushes down on the rear of it. If there is tilt control acting through the roof unit, there may not be a problem with vertical force. But if the tilt control acts at the point at which the carbody is fastened to the truck, the axle itself may be lifted, posing a derailment potential—particularly given that the IRWs on these cars are less forgiving of L/V ratios in general. The vertical buckling effect is illustrated in Figure 2.

Figure 2. Buckling can also occur in the vertical direction. This can pose derailment potential, particularly on cars with IRWs.

A similar situation can occur in acceleration as well. The trailing axle of the center section will be lifted in that case. The two buckling effects do not occur in isolation, of course. The same action can cause both effects; they can, and do, occur simultaneously. The wheels in the center section, therefore, are lifted and forced sideways at the same time, even on tangent track. This, combined with any other design factors that might be generating high L/V ratios, can lead to a very high propensity for derailment—and one that is very difficult to predict analytically.

Derailments that occur due to these effects can be particularly difficult to identify after the fact and a false sense of security may well be generated if a minor track perturbation is identified at or near the scene and falsely identified as the cause. The problems described here can be exacerbated if a disabled vehicle is being pushed, especially through a curve.

Multi-body cars have been in service on various types systems for many years. The vehicle/track incompatibility issues associated with the introduction of single-articulated cars more than 30 years ago have been resolved. The introduction of multiple articulation cars with short center sections and non-swiveling trucks, and the introduction of low-floor vehicles with IRWs, in particular, have created a new set of vehicle/track interaction issues that require a new round of technical analysis to ensure compatibility and safe operation on existing North American track.

Roy Smith is President of RESCO Engineering, a railway dynamics consulting and design organization in Kingston, Ontario.

This article is based on "Considerations for Operation of Low-Floor Vehicles on North American Track Designs," a presentation by Roy E. Smith, President of RESCO Engineering, at Interface Journal and Advanced Rail Management's Rail Transit '05 Wheel/Rail Interaction Seminar.

< PREVIOUS PAGE | PAGE 3 OF 3