Adequate lateral resistance is required to provide the stable track structure necessary for safe rail operations on passenger and freight railroad track. Insufficient lateral resistance, coupled with a large thermal compression force in the rail from high rail temperature, can buckle the track structure. Railroads typically use mechanical stabilization, slow orders, or both, following maintenance operations that disturb the ballast section, such as track surfacing and alignment. Tests were conducted to improve the understanding of lateral resistance variations on concretetie track caused by surfacing and subsequent stabilization or compaction. Factors influencing track stability are summarized, maintenance procedures are described, the single-tie push test is described, and test results are presented. Tests were conducted to evaluate the changes in lateral resistance, from the trafficked, well-consolidated track structure before surfacing and alignment through the laterally weak track structure after surfacing. The influence of stabilization on the lateral resistance of the track structure was evaluated. The tests results indicate that surfacing significantly reduces the lateral stability of the track to a potentially critical level. Mechanical stabilization following surfacing significantly increased the lateral stability of all sections tested.
Maintaining high, stable rail neutral temperatures helps prevent the buckling of continuous welded rail (CWR) track. Rail neutral temperatures are typically set high during installation (90°F to 110°F), but the large variations that develop during revenue service often lead to buckling-prone conditions. Readjusting or correcting for these variations requires CWR to be destressed with the use of procedures that do not always restore the desired target neutral temperature. As part of the Federal Railroad Administration's Track Systems Research program, the U.S. Department of Transportation's Volpe Center is investigating rail force and neutral temperature influences on track buckling. An analytic model for field applications has been developed to improve destressing and readjustment of CWR in both winter and summer conditions. The model has been validated in several field tests on instrumented CWR test segments under both high tensile and compressive force conditions. Both wood and concrete tie tracks were tested, and the rail longitudinal movement, rail gap, rail force distributions after rail cutting and welding, and readjusted neutral temperature were measured and correlated with the model predictions. The model and test results were used to develop a field tool for more effective destressing and readjustment of CWR. The tool provides the required removal lengths of anchors/fasteners, the rail gap size requirements when mechanical loads (rail-pullers) are used to adjust to the desired neutral temperature, and the required amounts of steel removal in summer when cutting rail out for stress relief.
This analysis provides information on the distinguishing characteristics of the drugs seeking this designation and the decisional factors used by CDER to either grant or deny breakthrough therapy designation requests. This paper provides greater transparency into the CDER decision process, so the public can better understand how breakthrough therapy designations are determined.
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