Interface - The Journal of Wheel/Rail Interaction

EFFECTS OF WIDE GAUGE

Effects of Wide Gauge on Derailment Potential (continued)

Wheelsets are pressed on the axle with a back-to-back dimension of nominally 53 inches. Given that the wheel tread area is approximately 3 inches wide, if the gauge widens by about 3 inches, the wheel tread can slip across the head of the rail and drop down. This would indicate that 59 1/2-inch (56 1/2 + 3 inches) gauge is the safety limit. However, if the flange is worn on the opposite wheel, 2 1/2 inches of gauge widening may be sufficient to allow wheel drop, or a total gauge of 59 inches (56 1/2 + 2 1/2 inches). Thus, pre-existing wide gauge in the range of 58 inches means that only an additional inch of dynamic rail movement could result in wheel drop. This is one reason that the FRA limits maximum gauge for Class 1 track to 58 inches. For Class 3 and 4 track, the maximum allowable gauge is 57 1/2 inches.

The FRA gauge standards from Section 213.53 of the federal Track Safety Standards are presented in Table 1.

Wide gauge also affects the ability of wheelsets to steer around a curve. A new and properly tapered wheelset moves to a larger diameter on the outside rail of the curve, and a smaller diameter on the inside of the curve. This creates a radius differential of several millimeters, and a net steering moment on the wheelset. In the real world of railroading, many wheels become hollow worn, creating a false flange on the outside edge of the tread. As gauge becomes wider, the probability of the false flange being pulled up onto the head of the inside rail becomes greater. The false flange is often a larger diameter than the flange root area, and consequently creates a reverse steering moment for the wheelset. If the wheelset doesn’t steer, the truck warps, and gauge spreading forces increase even higher. Thus, pre-existing wide gauge in a curve has the two-fold effect of 1) lowering the safety margin for wheel drop, and 2) increasing the gauge-spreading forces that lead to gauge widening. In academic terms, you get the "double whammy."

Part 2 of "Effects of Wide Gauge on Derailment Potential" will explore the many causes of "wide-gauge" derailments, and how to spot the telltale signs of impending wide gauge. Part 2 also will examine instances in which "wide-gauge" derailments may not be the result of a track problem.

Gary P. Wolf is President of Rail Sciences Inc.

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