The primary objective of this research was to validate the finite element (FE) stress levels obtained during previous research through physical testing of a selected knuckle design. The validated FE model was used to investigate potential improvements in knuckle fatigue life, such as higher tensile strength material and design changes to critically-stressed knuckle locations. 
Researchers found that the fatigue life of a coupler knuckle can be improved nearly 400 percent by increasing the tensile strength for M-201 Grade E steel from the present Association of American Railroads minimum of 120 ksi to 125 ksi. Increasing the thickness at two key locations in the studied knuckle increased the fatigue life by 83 percent even for the minimum tensile strength of 120 ksi for Grade E steel. A design of experiments approach for optimizing the knuckle based on the tensile strength, thickness of the inner face, and spacing of the cores at the flag hole is recommended. This method will find the optimum design parameter changes to achieve the best knuckle fatigue life possible under the geometry and weight constraints.