Track Buckling Research
Download Track Buckling Research in PDF format (986KB).
1.0 Introduction
This research is intended to improve railroad safety by developing means to prevent derailments due to lateral buckling of the track under a moving train.
1.1 PROBLEM DEFINITION
Track buckling is formation of large lateral misalignments in continuous welded rail (CWR) track, often resulting in catastrophic derailments. Both curved and tangent tracks are susceptible to buckling with typical curve buckle amplitudes ranging from 6"-14" and tangent buckles from 12"-28". Buckles are typically caused by a combination of three major factors: high compressive forces, weakened track conditions, and vehicle loads (train dynamics).
Compressive forces result from stresses induced in a constrained rail by temperature above its "stress free" state, and from mechanical sources such as braking, rolling friction and wheel flanging on curves. The temperature of the rail at the "stress-free" state is known as the rail neutral temperature (i.e. the temperature at which the rail experiences zero longitudinal force). Initially, the rail's installation temperature or "anchoring temperature" is the rail's neutral temperature. Hence, at rail temperatures above the neutral, compressive forces are generated, and at temperatures below the neutral, tensile forces are developed. Track maintenance practices address the high thermal load problem by anchoring the rail at (neutral) temperature of 95 -110 F. This high neutral temperature range prevents the generation of excessively high buckling forces even when the rail temperatures reach 130 -150 F.
Weakened track conditions impacting the tracks buckling potential include: reduced track Resistance, lateral alignment defects, and lowered rail neutral temperature. Track resistance is the ability of the ballast, ties and fasteners to provide lateral and longitudinal strength to maintain track stability. Resistance is lowered if ballast is missing from under the ties, in the crib or from the shoulder. A full ballast section is important, especially on curves. Adequate ballast in the high side in curves should be on the order of 12"-18" to provide adequate lateral strength. Ballast on the low side is important because inward (pulling-in) movement in cold weather could lead to line defects and lowering of neutral temperature which could lead to a buckle when higher temperature rises occur in early spring. Track resistance is also lowered when ballast is disturbed. Surfacing, tie renewal and undercutting operations will weaken ballast resistance by as much as 40%-60% of undisturbed track. It is a usual industry practice to restrict train speed to minimize train forces while ballast strength is being restored either by traffic or by mechanical consolidation means. Longitudinal resistance offered to the rail/tie structure by adequate rail anchoring is important to prevent rail running and hence the decrease of rail neutral temperature.
Lateral alignment defects also reduce the track's buckling strength because buckles tend to initiate at alignment deviations. The larger the line defect, the more buckling prone the track will be. Alignment errors must be corrected in hot weather and in early spring when curves tend to realign themselves from a winter "pull-in" condition. Buckles can also initiate at bad, crooked welds.
Maintaining a stable and high rail neutral temperature is critical for buckling prevention. Neutral or force-free temperature of CWR is usually different from initial installation or anchoring temperature. This difference is attributed to several factors, including rail longitudinal movement, track lateral shift/radial breathing in curves, track vertical settlement, and maintenance activities. Rail longitudinal movement (creep) is due to train braking and traction forces, or to differential thermal forces (sun and shade). Track lateral shift can be caused by excessive truck hunting, and by lateral forces generated by curving or by lateral misalignments. Compressive and tensile forces can cause radial breathing of curves especially in weak ballast conditions. Vertical differential settlement of rails can occur on new or recently surfaced track, or in areas of weak subgrade conditions. Maintenance operations influencing neutral temperature changes include: lifting, lining, and tamping, replacing broken rail, destressing, and installing CWR in cold weather. Research to date has shown that typical CWR rail installation (stress-free) temperatures of 100F can reduce in service to 50 - 60F due to these effects.
Track buckles usually initiate at small alignment deviations. Wheel loads and train action (dynamic uplift wave) tend to increase its size to levels which trigger the buckling process. Most buckling derailments tend occur deep in a train. Vehicles contribute to buckling by exerting lateral wheel forces in a curve. Lateral forces can also occur in tangent track from car movement caused by line or surface deviations or track hunting. The track must absorb this energy. Slack action, heavy dynamic braking and emergency brake applications can trigger a buckle. It is important to inspect track after a train passage in hot weather, especially if the track has recently been disturbed.
The above is a brief summary of the track buckling problem in terms of the three major causal factors: high compressive forces, weakened track conditions, and vehicle loads (train dynamics).
1.2 PROBLEM SEVERITY
Illustrations of track buckles are shown below together with the number of buckled track derailments/damage over the past ten years. As can be seen, the past five years' statistics indicate an average of 38 derailments a year with an increasing yearly damage level to as high as $17 million in 2002.
Track buckling illustrations and accident statistics
Currently there are no FRA safety performance standards in place addressing CWR buckling safety, the development of which is the principal objective of this research program. In its most recent Track Safety Standard promulgated under CFR 49/213.119 and 343 in 1998, FRA does however require railroads to have procedures in place for the safe installation, adjustment, maintenance and inspection of CWR. Key parts of these procedures address the adequacy of slow-order applications at high ambient temperatures and after track maintenance, and in providing adequate neutral temperature control when repairing/destressing CWR. The development of new data and information on these elements to support FRA standards and industry practices are also key parts of this research program.
1.3 RESEARCH TO DATE
The FRA and Volpe Center has been conducting research to better predict risk of catastrophic derailments due to sudden lateral buckling of track. This research has developed the rationale and approach to buckling prevention which consists of:
- the prediction of critical forces and conditions leading to buckles, and
- using a diagnostic tool to measure the in-situ forces against an "allowable" value.
Specific research activities geared toward this included developing predictive tools for evaluating the likelihood of track buckling in CWR tracks, applying these tools for buckling safety assessments i.e. toward determining the "allowable" values, evaluating the effectiveness of industry maintenance practices and procedures for the safe management of CWR, and developing techniques to measure longitudinal forces in CWR. This research has also identified the many parameters/conditions influencing CWR track buckling safety, the three most important ones of which are summarized below:
| Rail Longitudinal Force (RLF)/Neutral Temperature
|
 |
|
|
|
