Interface - The Journal of Wheel/Rail Interaction

REDUCING DERAILMENTS

Reducing Broken Rail Derailments in Dark Territory
(Part 2 of 2)
By Norman Hooper • April, 2008

This article reflects observations made while serving as Chief Engineer of BC Rail, a Class II railway (now owned by Canadian National) during a period in which the railway reduced service failures and broken rail derailments in dark territory by 60% — an improvement owing to treating the wheel/rail interface as a system, and better managing the risks. Part 1 of this article examined risk management procedures that have shown effective in reducing derailment potential.

As Part 1 of this article indicates, derailments are system failures — a combination of Operating, Mechanical and Track issues. Management must probe to understand all areas that contribute to failures in order to make a correction. Toward that end, maintenance practices on railways in signal-less “dark territory ” — territory in which there is no signal system to alert operators of the likelihood of a broken rail — must be closely monitored, particularly during the onset of cold weather.

Rail temperature plays a crucial role in resistance to track buckling derailments. Railways have been pushing the high end of the Preferred Rail Laying Temperature (PRLT) to prevent track buckles or “sun kinks.” Some Class 1 railways have a PRLT of 80 degrees F and allow +10 degrees F; others have a PRLT of 90 degrees F and allow as much as +25 degrees F. Allowing a high PRLT can avoid the cost of de-stressing rail that has been laid during daylight hours in the summer, but it can also increase risks. While rail’s neutral temperature has a tendency to drop over time, significant differentials from the rail’s neutral temperature, which occur during cold winters, can rapidly accelerate the growth rate of transverse defects (TDs). Railways operating in dark territory, in particular, must carefully assess the risks associated with the PRLT range.

Railways operating in dark territory should review their Standard Practice Circulars to ensure that they address the onset of winter. When that hard, cracking cold weather comes on, the entire track system should be inspected as the temperature falls, with particular attention paid to known defect “hot spots.” Most pull-aparts occur during the first sustained cold snap as cracked joint bars and hidden rail defects reveal themselves (see Figure 1). The first cold snap is also when problems with hi-rail trucks and snow clearing equipment are likely to occur. Avoiding or preparing for these types of problems should be part of a railway’s winter plan. The frequency of track inspection should also be reconsidered and increased, if possible, during the onset of cold weather, and as the temperature plunges for the first time that winter season.

Residual Stress

Residual stresses in rails can generate significant problems at the most inopportune times. One type of residual stress in rail arises during the manufacturing process. In some steel mills, the rail comes out of the heat-treatment process shaped a bit like a banana. When the rail is run through a roller-straightener, the top portions of the head and base are “cold-worked” to make the rail straight. (The steel must be stressed enough to permanently yield.) The steel beneath the yielded portion contains high residual/internal stresses. The higher the strength/hardness of the steel, the higher the residual stresses can be. These stresses don’t exist at the free end of a new rail; it is only several feet into the rail that the straightening forces were applied. The web of the rail in Figure 2 was machine cut for 18 inches, and the relative horizontal and vertical movement of the rail ends were measured. When a wedge was placed into the cut and tapped, the rail ripped open, nearly to the end of the section due to the residual stresses.

Cracks that develop in the web of these rails can accelerate catastrophically. (For more information on residual stresses see Parts 1 and 2 of “Understanding Stresses in Rails”) The sources of cracks can include rough welds, drilled bolt holes, torch cutting, damage from spike mauls, stamping, or weld bonds.

The current AREMA specifications relating to residual stresses in rail are not comprehensive. They represent a compromise in what all mills could make and what could reasonably be tested. They don't address the stresses that are added when the rail ends move laterally. They don't assess closure for compressive stresses. In BC Rail’s experience, rail that meets the current AREMA specifications can still have a residual stress failure. The rail in Figure 3 was recovered from a derailment. The initiating point was a slight surface roughness on a flash butt weld near the neutral axis. Instead of an “S” or horseshoe failure, the crack ran down the web.

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