A train almost veered onto the wrong track after a line fault at Cardiff Central, Network Rail said.
If the driver had not stopped, the carriage would have been diverted onto a line where trains could run in both directions.
Network Rail said it had launched an investigation into the 'serious signalling irregularity' at Cardiff East Junction.
No-one was injured during the incident on 29 December 2016.
The incident occurred after a section of track was reopened following resignalling and track remodelling work in and around Cardiff Central station.
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