A train carrying eight passengers and two staff was derailed between Brithdir and Bargoed because of a landslip due to flooding and thawing snow, according to Network Rail.
The 6.10am Rhymney to Barry Island service, the first of the day, was derailed just before 6.30am.
The Network Rail spokesman said: “At 6.28am the driver of the 6.10am Rhymney to Barry Island service, formed by a two-car unit, reported that the train had struck a landslide, with fallen trees therein, and become derailed by the leading bogie.
“The accident occurred on the single line at a location about 700 yards on the northern approach to Bargoed from Brithdir.”
There were three reported injuries. Two people sustained bumps to the head and one person suffered shock. All injured persons were treated on the scene by ambulance crews and released.
The spokesman added there was significant debris at the site and that the Rail Accident Investigation Board has been informed.
A total of seven train units are trapped at Rhymney, although Arriva Trains Wales has said it has enough to carry out its services. Trains will be terminated and restarted at Bargoed.
All eight passengers have been rescued and have been moved from the site by a rescue train.
Road transport, operated by New Adventure Travel, is being arranged to operate between Rhymney and Bargoed for the rest of the day.
Removal of the landslip has begun and it is hoped to re-rail the train once this is completed.
If all goes well, it is hoped to re-open the line with a 10mph speed restriction by 12 noon on Thursday January 31.
I was shocked to see this, I have been on the Arriva trains to Cardiff on that Rhymney line, so it's worrying! I'm glad though that nobody was injured.