Real-Time Train Tracking over Satellite Backhaul: A GNSS/INS System with HMM Map Matching and Graph-Aware Dead Reckoning for the Greek Railway Network

Modern railway operations require continuous, high-accuracy train position awareness for monitoring and passenger information. The 2023 Tempi rail disaster exposed major gaps in Greek railway detection infrastructure, with ~80% of the network lacking ETCS coverage. This paper presents a real-time train tracking system deployed across the Greek railway network as a rapid overlay requiring no trackside modifications.The system combines Trimble GNSS/INS receivers with HEPOS RTK corrections to achieve centimeter-level accuracy in open-sky conditions, addressing the challenge of parallel tracks spaced only 3–4 m apart versus typical GNSS errors of 5–10 m. Connectivity is provided via Starlink terminals over a WireGuard VPN mesh with an Azure hub, using TELTONIKA industrial routers onboard trains.
Five key contributions are introduced. 1) An HMM-based map-matching algorithm with RTK quality gating, allowing only high-quality fixes for track-switching decisions. 2) A graph-aware dead-reckoning engine that constrains movement to rail topology, halting at switches and handling tunnels. 3) A client-side animation pipeline inspired by game networking, using jitter buffering, PLL timing, EMA clock sync, and Kalman filtering to ensure smooth motion despite 20–600 ms latency. 4) An anticollision module using switch-count logic, pathfinding, and braking models to generate warnings without interlocking data.5) A microservices architecture using Pub/Sub and SSE to deliver sub-second updates to the frontend. 
Deployed on the Athens–Thessaloniki corridor (~482 km), the system achieved <0.5 m accuracy (RTK-fixed), <5 m in degraded modes, and reliable dead-reckoning in tunnels up to 2 km. It delivers 60 fps visualization with <1.2 s latency. The system is for situational awareness only and is neither a safety-critical or certified signaling solution, nor a replacement for ETCS or interlocking systems.
The system is currently fully operational at the government lvele, under reeal production mode, available as https://railway.gov.gr