Interface - The Journal of Wheel/Rail Interaction

DERAILMENT PREVENTION

Inspection and Analysis of Switch Derailments
(Part 2 of 2)
By Gary Wolf • October, 2006

Part 1 of this article examined trailing-point derailments, facing-point derailments and thin flange wheels. Part 2 examines wheel climb and frog-related derailments.

Crosslevel variances in turnout geometry represent a common cause of wheel-climb derailments. It’s not unusual to find bolted or insulated joints within 30 to 40 feet of switch points. This is particularly true in signaled territory where insulated joints are used. As with any set of bolted joints, mud pumping can occur at these joints. This pumping can cause a crosslevel twist or variance over the truck centers of the car, such that as the rear trucks are traversing the low joint, the opposite diagonal corner on the leading end of the car may be traversing the point rail in a reverse move into the turnout.

Depending on the amount of twist, some degree of vertical wheel unloading will occur on the lead truck. At the same time that the wheel is losing vertical load, the lateral forces on the point rail may be reaching a peak value. The lateral forces developed on the point rail are substantial. The point rail, typically a section of straight rail planed to a fine tip, represents an abrupt change in direction, causing excessive wheelset angle of attack and attendant high flanging forces. Any reduction in vertical load at this point, coupled with a high lateral force, will result in a high L/V ratio with a high probability of wheel climb. Figure 1 shows a diagram of a potential track twist condition entering a turnout.

In a similar sense, the lead truck of a car may be positioned 40 to 50 feet beyond the point rail, and due to a crosslevel twist, the rear trucks may experience a similar loss of vertical load across the diagonal corner of the truck just as the wheels are traversing along the point rail. We often see this condition on industry sidings where the tracks slope away from the main line down to an industry. Figure 2 shows an example of an industry lead track where the curved stock rail is clearly lower than the closure rail.

A mechanical condition that might contribute to wheel climb on the point rail is tight side bearings. A car with tight side bearings cannot accommodate even nominal track twist without causing significant loss of vertical load, especially across opposite diagonal corners of the car. Stiff trucks resulting from conditions such as fouling around the bowl rim, a twisted center sill, or a dry centerbowl, can also contribute to wheel climb.

Track geometry should be measured whenever a wheel climbs a point rail. The track geometry should be measured using 15.5-foot stations, accurately checking crosslevel variation at every station and every joint in the turnout, especially the heel block. Cars involved in wheel climb should be inspected to determine if any mechanical deficiencies are present – specifically, insufficient side bearing clearance. In addition to checking crosslevel, the horizontal alignment deviation should also be checked using a 62-foot chord. Sharp changes in horizontal alignment, especially in the area of the closure rail, can contribute to excessive wheelset lateral force.

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