A train which derailed near Bargoed just before 6.30am this morning struck debris from a landslip, Network Rail has confirmed.
Around 10 passengers were aboard the 6.10am Rhymney - Barry Island service when the incident occurred between Brithdir and Bargoed.
There are no reported injuries.
The driver reported that the train had struck a landslip, with fallen trees therein, and become derailed by the leading bogie.
Reports from site suggest that there is significant debris in front of the train which will require clearing. The Welsh Ambulance Service is on site. The fire brigade is en route to assist if required.
Plans are in place to rescue the passengers using a train in rear with a mobile operations manager on board. This unit should be able to attach to the rear of the derailed unit as they are compatible.
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The decision comes after a campaign led by Plaid Cymru and Rhondda Cynon Taf councillor Elyn Stephens to eradicate period poverty.
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