Interface - The Journal of Wheel/Rail Interaction

REDUCING DERAILMENTS

Reducing Broken Rail Derailments in Dark Territory (continued)

Experience has shown that testing equipment doesn’t always find defects. Research by David Jeong and the Volpe Center on analytical modeling of rail defects is pertinent to rail defect management (see Figures 3, 5 and 6). (The Volpe Centre has a tremendous body of research relevant to railway field personnel.) As shown in Figure 3, the smaller the defect, the less likely that testing equipment will find it. There are things that can be done in the field, however, to improve the probability of detecting defects. These include cleaning up the rail by grinding or turning down the lubricators prior to testing so that the operators can get a better look. If the rail is greasy or shelled out and the operators are told to eliminate “false-positives,” the net result may be an increase in service failures. AREMA guidelines recommend accuracy of 65% to 95% for rail testing equipment, depending on the size of defects. Accuracy of 65% isn't great. At this rate, a defect doesn’t have to be very big to result in a service failure in cold weather conditions. Because of this, it pays to clean out small bolt hole cracks and to turn up the gain a bit in order to reduce the risk of a service failure when testing branch lines in dark territory.

It also pays to review the test data where service failures have occurred in order to see how the railway field staff and rail test contractor can improve. Rail testing is art as well as science. There's a wide range of testing equipment from different contractors and suppliers; some of it may be better suited than others. Operators should be encouraged to assess their findings whenever there is a spike in the number of detected defects. They should be encouraged to ask: “Why are these defects bigger? Why weren’t they caught last time? Relative to the tonnage interval, should we have found them last time?”

In order to reduce the risks associated with defect testing, railways should:

• Have rail testing equipment dedicated as a “first call” to their railway, or own their testing equipment. As a smaller railway, the rail testing contractor came to BC Rail after the big railways had finished their programs. Sometimes, it was too late. This led BC Rail to purchase its own equipment. We paid for it in the first year. While owning the equipment (or sharing it with a few other railways) creates work because of equipment maintenance and operator training/quality, it provides flexibility in being able to vary testing schedules according to risk. Stand-by costs for weather or traffic are much lower as well.

• Test the isolated hot spots more frequently. Particular areas — such as curves in which the number of TDs is increasing, or an area with 85-pound rail in which the tonnage is increasing — should be tested more frequently. You might be testing only 4 track miles of curves on an entire subdivision, but it’s where your troubles are.

• When testing, test hot spot curves in both directions. On BC Rail, we found that we increased the percentage of small TDs that were detected when we turned the equipment around and ran it the other direction.

• In northern areas, it’s wise to perform testing in the late fall and winter. Nobody wants to change defective rail out in December, but service failures on BC Rail typically occurred with the onset of cold weather (and in the early spring). And while changing out rail in the winter is a pain, it’s not as bad as cleaning up a wreck.

Rail and the Service Environment
When David Jeong and the Volpe Center modeled detail fractures, they looked at the growth rate sensitivity of different environmental factors. They examined the effect of a temperature differential of as much as 110 degrees on rail that broke at 52 MGT on average. They examined the effects of wear, contact location and the weight of the rail on defect growth.

Their study showed that crack growth life — the time it takes a defect to grow through 80% of the head — was significantly influenced by temperature. It showed that a rail temperature differential (from neutral) of between 0 degrees F and 110 degrees F could reduce the service life of the rail from 110 MGT to 10 MGT. This is significant on any railway, but particularly on those operating in dark territory. When there is no signal system to warn of a rail break, temperature changes, residual stress, the amount of curvature, the rail type, the axle load, wear, etc., must be taken into account when performing a risk analysis. Railway engineering/maintenance managers also must determine the frequency of testing, the type of rail and rail testing equipment to invest in, and the maintenance practices that are best for their railway.

References
Jeong, D., “Analytical Modeling of Rail Defects and its Applications to Rail Defect Management,” Volpe National Transportation Systems Center, January 2003.

This article is based on a presentation made at Interface Journal and Advanced Rail Management’s 13th Annual Wheel/Rail Interaction Seminar, May 2007.

Norman Hooper P.Eng., is Project Engineer, Hatch Mott MacDonald.

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