Interface - The Journal of Wheel/Rail Interaction

TRACK BUCKLE

Preventing Track Buckle (continued)

A sufficient ballast section is also important to prevent the track from moving longitudinally. A full ballast section in the crib area provides resistance to tie bunching. Figure 5 shows the bunching effect due to an insufficient ballast section. Ballast shoulders are important to control the tendency of track to buckle in a lateral direction. A ballast shoulder of 6 inches is adequate to prevent lateral movement on tangent track, and a shoulder of 12 inches is generally recommended for sharper curves. In addition to the proper quantity, the quality of the ballast is important. Ballast that has been broken down into small particles with rounded edges does not interlock and provide the necessary compressive strength. Ballast that is heavily fouled with fines and mud is not effective and allows heat-induced stresses in the rail to move the track sideways.

In addition to a sufficient quantity of ballast, the ballast should be firmly and thoroughly compacted in order to reach its maximum holding strength. Whenever track is disturbed through tamping, tie insertion, re-gauging or re-alignment, the compaction of the ballast is destroyed and track stability lowered. Some railroads use vibration type compactors to re-establish the ballast compaction. Other railroads issue slow orders and watch several trains over the disturbed ballast site before allowing trains to operate at track speed. Research has shown that the track does not reach maximum compaction until one-half to one million tons of traffic passes over it.

Checklist for Spring Inspection

With the approach of warm weather each year, track maintenance personnel should:
• Note any locations where plug rails were installed in the winter. Determine the installation temperature, and adjust the rail by cutting out an appropriate amount of rail.
• Check for signs of heat stress, or nervous rail. Rail that alternately crowds the shoulder of the tie plates is under great stress and is trying to buckle.
• Check anchor patterns and anchor integrity. Indications that rail is pulling through the anchors is a sign of excessive heat stress (see Figure 6). Ensure that anchors are tight against the surface of the crossties. Proper anchoring is especially important in areas where trains require high tractive or braking effort, such as on ruling grades.
• Check ballast quality and quantity. Ensure that shoulders and cribs have the sufficient quantity of ballast to restrain the lateral buckling forces in the track structure. Pay special attention to areas around bridge approaches where longitudinal stresses are highest. Look for evidence of ballast voids or pockets. Pockets forming on the ends of ties are a telltale sign of lateral track movement due to heat stress.
• Check for proper ballast compaction after maintenance activities. Issue slow orders when required and ensure compliance with the slow orders.

With proper precautions and inspection track buckles and resulting derailments can be prevented.

Gary P. Wolf is President of Rail Sciences Inc.

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