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Switch Point Derailments: Is it the point or the wheel?
(Part 1 of 2)
By Gary Wolf • July, 2006
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No other type of derailment causes as much angst among railroaders as the switch point derailment.
The switch point derailment can take several forms, but it generally boils down to a disagreement between the Mechanical Department that blames the switch point, and the Engineering Department that blames the wheel. It may surprise you that it is possible that neither is to blame; the cause may lurk elsewhere. Following are some of the important issues that must be investigated after a switch point derailment in order to get to the root cause.
First, it is important to determine exactly what happened at the switch point. Several possibilities exist:
• A wheel flange may strike the tip of the point (especially if the tip is cracked or blunted) and climb up the tip of the point, run along the top of the point rail, and fall off the point rail near the heel block.
• A thin wheel flange (not necessarily condemnable) may split the switch point, forcing the point open, and running down along the back of the point and between the gauge face of the stock rail. This is generically called a “split switch”.
• A wheel may climb the point somewhere between the tip of the point and the heel block, eventually falling off the point rail near the heel block.
• In a trailing point move, a wheel may spread gauge at or near the tip of the point, dropping between the rails.
The root cause of these different wheel actions may vary widely and may be due to mechanical conditions, track conditions or human factors. Figure 1 shows the result of a typical split switch derailment.
Trailing point moves
Let’s first deal with trailing moves through the point area. Engineering practice dictates that the point rail is maintained roughly 1/4-inch higher than the stock rail along its length and at the convergence between the point and the stock rail. This feature allows the wheel tread to be gently lowered from the top of the point rail onto the top surface of the stock rail as the point and the gauge face of the stock rail converge. If the point rail is lower than the stock rail, the outside edge of the wheel rim can engage the gauge corner of the stock rail, forcing the gauge open by rolling the stock rail outward, allowing the wheel to drop in (see Figure 2). This possibility is exacerbated if the wheel tread is moderately or extremely hollow worn with the development of a “false flange” on the outside edge of the tread. As of 2005, the AAR set a hollow wear limit for wheel treads of 4 mm.
