A new multiscale computational method predicts RCF crack formation in rails by integrating multi-body simulations, finite-element methods, and critical plane approaches. Multi-variable modal value sampling is introduced to reduce the computational expense of large-scale FE studies by capturing multiple wheel passes in a single loading profile. The Smith-Watson-Topper (SWT) critical plane fatigue damage parameter is selected to capture the non-proportional multiaxial load history drivers for RCF formation. The critical plane fatigue damage parameter matches the experimentally observed location and orientation of RCF cracks, highlighting the applicability of this new method for future RCF assessments.