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.
More top news
Gareth Bale put any domestic troubles behind him as he starred in Wales' emphatic 3-0 win over Israel in Haifa.
National Trust Wales has announced it's to open the new Far End Garden at the end of this month, spanning 80 acres of Snowdonian hillside.
Cardiff's tourism information centre is closing today, with the only staffed provision residing in Cardiff Bay.