As railway mobile communication systems rapidly evolve toward digitization and intelligence, the traditional multipath-interference optimization model of narrowband GSM-R can no longer satisfy modern rail-transport demands for highly efficient, accurate and intelligent operations and maintenance. This paper first explains the mechanism and scenario-specific features of GSM-R multipath interference, and then analyzes the core modules and overall architecture of a ray-tracing-driven high-speed-rail wireless-network optimization system. Taking the voice-quality degradation that occurs inside a mountain tunnel on the Wuhan—Guangzhou high-speed line as an example, the system is employed to pinpoint the root cause of the poor voice quality. Three network-optimization schemes are proposed and simulated. The results show that by adjusting the optical-fiber repeater's transmit power and antenna downtilt in the multipath-interference zone, the interference at the onset of voice-quality degradation is significantly mitigated, providing a reference solution and technical foundation for the digital, high-quality optimization of GSM-R multipath-interference problems.