ETCS wrong-side failure in Wales: wrong wheel diameter left a 610 m position error

An ETCS-equipped train in Wales stopped at the end of its movement authority while its cab display still showed 610 metres left to run, because the onboard system was using the wrong wheel diameter to calculate its position. The Rail Accident Investigation Branch (RAIB) published the findings of its preliminary examination of the incident, a wrong-side failure near Welshpool on the Cambrian lines, on 21 October 2026.

The incident happened at around 23:52 on 15 September 2026. An infrastructure monitoring train was running under ETCS Level 2 supervision when it came to a stand at the block marker that marked the end of its movement authority. At that point, the onboard equipment was telling the driver that the train still had 610 m to go before reaching that limit.

According to RAIB, the onboard equipment was working with incorrect wheel diameter information. Following routine maintenance, a component of the onboard equipment had prevented the correct value from being recorded. ETCS calculates how far a train has travelled from the rotation of its wheels, so a wrong diameter distorts the distance it believes it has covered. The independent speed measurement system and the trackside balises, which are meant to detect and correct this kind of error, did not correct it.

RAIB has decided not to carry out a full investigation. Instead, it has written to Network Rail to make it aware of the circumstances and of potential weaknesses in the safeguards of the onboard ETCS equipment, so they can be considered as part of Network Rail's own investigation. Copies have gone to the Office of Rail and Road and to the operator of the monitoring train. RAIB noted that it has previously investigated ETCS integrity issues on the Cambrian lines.

Our analysis

This is exactly the kind of failure ETCS is designed to prevent. Odometry, the system's estimate of where the train is, always drifts, and the architecture relies on independent speed sensors and balise readings to keep that drift within a safe confidence interval. Here, a single wrong maintenance parameter went undetected by every layer. The lesson goes beyond Wales: as more fleets are fitted with ETCS across Europe and beyond, the maintenance processes that feed safety data into onboard units become as critical as the signalling itself. The next point to watch is what Network Rail's investigation concludes about how wheel diameter values are entered, checked and cross-validated after maintenance.

Sources: RAIB, "Wrong side failure near Welshpool" (21/10/2026); RailAdvent (October 2026); Rail Business UK (22/10/2026).

Photo: a Class 158 train in Transport for Wales livery. Peter Broster, CC0, via Wikimedia Commons.

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