Rail safety body investigates unexplained loss of signalling data

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RAIB, the Rail Accident Investigation Branch, is launching its own investigation of an incident last October in which drivers on a line in North Wales fitted with modern ‘ERTMS’ signalling were not warned of speed restrictions on the approach to level crossings. The cause of the failure has not yet been discovered.

During the morning of Friday 20 October 2017, a train driver travelling on the Cambrian Coast line reported that long-standing temporary speed restrictions were not indicated on the in-cab display. As signalling staff at the Network Rail control centre in Machynlleth investigated this report, they became aware that this failure applied to several trains under their control. The temporary speed restrictions were required on the approach to level crossings so that people crossing the line had sufficient warning of an approaching train.

The Cambrian lines were equipped in 2011 with a pilot installation of the European Rail Traffic Management System (ERTMS), a form of railway signalling. ERTMS removes the need for signals along the track by transmitting data directly to the train. This data is used to display movement authorities and other information such as temporary and permanent speed restrictions, on a screen in front of the driver.

Subsequent investigation found that the signalling system stopped transmitting temporary speed restriction data after a routine shutdown and restart at around 23:10hrs the previous evening. The signallers had no indication of an abnormal condition and signalling control centre displays showed these restrictions as being applied correctly.

The RAIB says it has decided to undertake an independent investigation because, to date, the signalling system supplier has not identified the cause of the failure.

The announcement added: “It is possible that finding the cause would have been assisted by downloading of suitable data from the signalling system before it was restarted during correction of the failure.”

The Cambrian Coast Line was the first in Britain to be fitted with ERTMS Level 2 signalling, which replaces lineside signals with in-cab displays. The system came into operation in 2011 following a period of testing, with equipment supplied by Ansaldo STS (now part of the Hitachi group). The route was chosen because it is self-contained and the previous signalling system was due for replacement anyway.

The installation was not without teething problems. At first drivers complained that displays fitted into elderly cabs that had little space for new equipment were unreadable in bright sunlight

Since the October incident an additional procedure has been introduced at the control centre that is intended to identify and avoid any recurrence of the failure.

The RAIB investigation will consider:

  • the geographic extent of the failure and the effect it had on the safety of railway operations
  • why trains were permitted to operate without information about temporary speed restrictions
  • practices for the gathering of data needed for investigation before restarting computer-based signalling systems after a potentially unsafe failure. 

The findings, including any recommendations to improve safety, will be published when the investigation is complete.

Network Rail now operates an ERTMS test facility from a base at Hitchin, Hertfordshire, with trains running on the Hertford loop line to ensure that lineside and in-cab equipment from different manufacturers all works together properly as intended.

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