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.
A train has been derailed by debris on the track near Bargoed following a landslip. Network Rail say around 10 people were on board at the time but nobody was injured. Buses are replacing trains between Bargoed and Rhymney.
The Welsh Secretary, David Jones, has expressed his pleasure at today's Network Rail announcement.
Mr Jones described a modern rail network in Wales as "crucial" for the growth of the country's economy.
As a regular rail user, I know that the re-signalling of many rail lines across Wales can only be good news for all passengers and I am particularly pleased that the important north-south line links will be some of the first to be upgraded. The Newport to Shrewsbury re-signalling also marks the start of the build programme for electrification of South Wales mainline which we announced last year.
– David Jones MP, Welsh Secretary
He also welcomed news that the mainline between London and Cardiff will be electrified by 2018.