An investigation into a “serious” signal failure in Wales is being carried out by Network Rail.
A train could easily have been diverted towards a line on which trains can run in either direction after a set of points were placed in an incorrect position during resignalling work at Cardiff East Junction.
A few minutes prior to the incident on 29 December 2016 another train had travelled over another set of unsecured points at the other end of the same crossover.
The investigation will examine the issues surrounding the incident, including events leading up to the commissioning of the new track layout and the management and team briefing process.
“The safety of passengers and our workforce is of vital importance and we are already conducting our own internal review of the incident,” said a Network Rail spokesperson.
“We will work closely with the RAIB as it completes its inquiry.”
The work was carried out as part of a signalling replacement scheme in and around Cardiff Central station, a project which has been in progress for several years.