This IDEA program study (HSR-41) confirms the results of its prior IDEA program study (HSR-24) that thermite-weld fatigue resistance can be significantly and reliably increased by improving the shape of the weld-toe in the web-base fillet and rail-base regions of the weld and by suppressing common weld-toe defects found there. Laboratory 4-point bending fatigue tests were carried out on five series of thermite welds: three series had a standard external shape currently in use and two series had an improved shape developed in this study. Test laboratory results show that improved welds have a fatigue life approximately 2 times that of standard welds. Twenty-one (21) welds with the improved shape were installed in the TTCI FAST track facility in Pueblo Colorado. Several of the 21 welds failed prematurely; and thus, all of the remaining 21 were removed from the track. A small fabrication fault in the web of the improved welds was identified and has been corrected, new molds are being manufactured and a second series of improved welds will be installed in the FAST track.