The Rail Accident Investigation Branch published a preliminary examination on 21 October 2026 into a wrong side failure of the European Train Control System near Welshpool, Powys, at around 23:52 on 15 September 2026. An infrastructure monitoring train operating under Level 2 ETCS supervision stopped at the block marker that should have marked the end of its movement authority, the report says. The on-board signalling equipment nevertheless told the driver that 610 metres remained before that limit.

The branch attributed the error to on-board ETCS equipment calculating the train's position from incorrect wheel diameter information. It said a component part had prevented the correct wheel diameter from being stored after routine maintenance. Neither the train's independent speed measurement system nor track-mounted balises, both intended to correct distance inaccuracies, proved effective in this instance.

RAIB said it had previously examined ETCS integrity issues on the Cambrian lines, including a 2019 report on the loss of safety-critical data. It concluded that a full investigation was not warranted, as wrong side signalling failures without an accident or near miss are normally handled by the industry. The branch said its findings pointed to potential weaknesses in on-board protection arrangements that merit further consideration by Network Rail and wrote to the company, copying the Office of Rail and Road and the train's operator.