This study investigates the effect of piston ring wear on gas and oil leakage in a locomotive reciprocating compressor using Computational Fluid Dynamics (CFD). The compressor is a critical component of railway braking systems, where any reduction in compressed air delivery directly affects braking performance and operational safety. The numerical model is developed using a combination of two-dimensional transient simulation for suction and compression processes and a detailed three-dimensional CFD model for the piston–ring–cylinder lubrication and leakage regions. Governing equations for mass, momentum, and energy conservation, along with compressible isothermal flow assumptions in narrow clearances, are employed to model leakage through piston ring gaps. The results indicate that piston ring wear significantly increases blow-by leakage and oil transport into the compression chamber. While increasing rotational speed reduces blow-by due to shorter leakage time, higher compression ratios intensify leakage due to increased pressure gradients and thermal–mechanical stresses. Validation against reference data shows approximately 10% deviation, confirming acceptable model accuracy. The study demonstrates that severely worn rings lead to substantial efficiency loss, even at high speeds, and significantly increase oil contamination in the discharged air. The findings provide valuable insight for improving piston ring design and optimizing maintenance strategies in locomotive compressors.