Accurate and reliable train localization is critical for rail safety, particularly on regional and rural lines where traditional track-based infrastructure (e.g., balises, track circuits) is often too costly. Global Navigation Satellite Systems (GNSS) offer a potential solution, but their performance degrades significantly in obstructed environments such as tunnels, forested areas, and deep cuttings commonly present on rail. This study presents a real-world case study of a GNSS-only navigation performance measurement on a regional railway track. Using a mass-market GNSS receiver and a high-precision reference system, the study analyses the position accuracy. Results highlight the limitations of GNSS-only navigation, particularly in meeting accuracy requirements for critical applications such as track distinction. To address these challenges, the study presents a comparative review of Alternative Positioning, Navigation, and Timing (A-PNT) methods. The technology level points to a multi-sensor fusion approach to ensure resilient, cost-effective rail localization for future intelligent and autonomous rail systems.