Interface - The Journal of Wheel/Rail Interaction

DERAILMENT PREVENTION

Switch Point Derailments: Is it the point or the wheel? (continued)

A point rail can wear down over time and become lower than the stock rail. Or, a stock rail might be changed out and the point rail left in service. In this situation, the point rail will be lower than the new stock rail. A good practice is to change the point rail whenever the stock rail is replaced. Also, Section 213.135(c) of the Track Safety Standards states

“Each switch shall be maintained so that the outer edge of the wheel tread cannot contact the gage side of the stock rail.”

Facing point moves

Most switch point derailments occur during facing point moves. If the point is new, the stock rail properly profiled, and the wheel tread relatively new, it is difficult to envision how a derailment ever occurs at the switch point because the wheel and the point have virtually no opportunity to interact. (See Figure 3 for an illustration of a new wheel operating on a new point.) But as we all know, in the real world, wear occurs on the wheels, points and stock rails. After wear sets in, it is easier to see how a split switch or switch point climb occurs. (See figure 4 for a view of a worn wheel on a nominal switch point.)

If a wheel flange splits a switch point, the switch point should be inspected to see if it is gapped due to improper geometry. This can be caused by a number of conditions, such as:

• The switch is out of adjustment.

• A condemnable thin flange wheel with heavy metal flow on the tip of the flange.

• The stock rail is worn lower than the point, exposing a gap.

• Heavy metal flow on the gauge corner of the stock rail is gapping the point.

• The switch has been run through and the switch machine damaged.

• There is lost motion in the switch machine due to wear.

• The stock rail is dynamically moving outward from the point due to inadequate bracing.

• The point rail is dynamically gapping due to pumping at the heel block that “see-saws” the point upward.

• A foreign object like snow, ice, rocks, piece of freight car, etc. lodges between the point and the stock rail.

• The point is chipped or broken, exposing a gap between the stock rail.

• Poor headblock ties or ballast support under the point allows the point to twist away from the stock rail.

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