This report is the second in a three-part series describing the technical contributions of the Federal Railroad Administration (FRA) and the Volpe National Transportation Systems Center (Vole Center) to the UIC/WEC (International Union of Railways/World Executive Council) joint research project on Rail Defect Management. Specifically, this report describes engineering analyses that are used to examine the growth of internal rail defects that were tested under field conditions. The first report in this series describes similar engineering analyses that were conducted to examine the growth of internal rail defects tested under laboratory conditions. The third report discusses some applications of the rail defect modelling work.