This report is the first 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 laboratory conditions. The second report in this series describes similar engineering analyses that were conducted to examine the growth of internal rail defects tested under field conditions. The third report discusses applications of the rail defect modelling efforts.