The study established a three-dimensional, thermal-kinetic-mechanical finite element (FE) model to simulate an additive manufacturing process with a laser powder deposition (LPD) approach for repairing a 75-lb rail that is broadly used for light rail transportation in the US. A worn rail specimen is repaired using 304L stainless steel powders as the deposition material for lab tests. The researchers incorporated an element-birth-and-kill technique to activate the deposition elements step-by-step, according to the build-up strategy along which the laser heat source is translated simultaneously. The laser power attenuation and solid-state transformation expressions are described using external user-defined subroutines for thermal and kinetic analysis, respectively. A set of equations for calculation of hardness for both of the rail and deposition materials are also defined and developed in the FE model. The microstructure distribution coming from kinetic analysis output is employed for hardness calculation. The estimation of the width and depth of the dilution zone in thermal analysis is compared with the experimental results of the repaired specimen to validate the thermal model. Scanning Electron Microscope (SEM) and Optical Microscope (OM) analyses, along with a Rockwell B-scale hardness test are performed to validate the outgoing microstructure and hardness results of the FE model. Mechanical analysis results showed that residual thermal stresses can significantly reduce the safety margin against shearing off the deposition part under a dynamic train load. The validated model demonstrated great potential for investigating the effects of the variation of different LPD process parameters on the final mechanical and metallurgical properties of the repaired rail.