The mainline railway into South Wales was shut because of an emergency.
There were no Great Western Railway or CrossCountry trains running into or out of south Wales following the incident between Severn Tunnel Junction and Newport on Saturday night.
Emergency services were attending the incident and all lines were blocked at Caldicot.
A spokesman from Cross Country Trains said: "Due to emergency services dealing with an incident between Severn Tunnel Junction and Newport (South Wales), all lines are blocked."
On its website, Great Western Railway says that disruption was expected until the end of the day. It's affecting all trains between Cardiff and Bristol on the mainline to London.
British Transport Police have been contacted for comment
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