Trains between Cardiff and Newport hit by signalling damage
Trains delays between Cardiff and Newport are being blamed on vandals who damaged signalling equipment on side of the tracks.
First Great Western said the line reopened after it was initially shut at around lunchtime on Friday.
Services were delayed but trains returned to normal running by around 15:30 GMT.
A replacement bus service ran between Cardiff Central and Newport, adding up to an hour to journeys.
The problem also affected Arriva and CrossCountry services.
Earlier, Network Rail said staff were working to restore the signalling.
Initial reports by the company that would-be cable thieves were responsible has been discounted.
British Transport Police (BTP) were called out at 12:02 GMT and the incident is now being treated as a case of vandalism.
A spokesman said: "BTP officers were called to the railway line near Dyffryn Estate, Newport, after Network Rail reported a loss of power to signals in the area.
"Officers attended along with engineers from Network Rail and discovered damage had been caused to lineside signalling equipment.
"The incident led to a number of trains being delayed or cancelled, causing disruption for passengers and the rail industry. The lines were fully reopened at 14:25.
"Inquiries are ongoing to trace those responsible for the damage."
South Wales Central assembly member Andrew RT Davies called those responsible "selfish, irresponsible louts".
Mr Davies, the Conservative leader in the assembly added: "This has caused complete carnage for commuters seeking to get in and out of Cardiff. It is simply unacceptable."
British Transport Police said anyone with information can contact the force on 0800 40 50 40, or Crimestkoppers on 0800 555 111.