The mountainous power grids exhibit significantly lower reliability compared to conventional urban grids due to inherent structural weaknesses, dispersed load distribution, and higher failure probabilities of power supply equipment. With the ongoing construction and commissioning of electrified railways in western China, it is crucial to analyze the impact of strong shocks and random fluctuations in traction loads on mountainous power grids, and to study the reliability enhancement of these grids considering electrified railways access, to ensure their safety and the reliable, continuous power supply for the railways. Therefore, this paper proposes a method to enhance the reliability of mountainous power grids considering electrified railways access. First, stochastic fluctuation characteristics of traction loads are simulated through train traction calculations. Subsequently, the reliability level of mountainous grids is quantitatively evaluated, with a novel line vulnerability index established to identify weak grid sections. Finally, two complementary enhancement strategies are proposed: dynamic line capacity expansion and optimized backup capacity allocation. Case studies demonstrate the effectiveness of the method through comparative analysis of reliability indices before and after implementation, confirming both technical validity and practical feasibility.