Crime and disorder on the railways in Wales has nearly doubled so far this year, according to rail company Arriva Trains Wales.
Services could be cancelled unless the situation improves
BBC current affairs programme Eye On Wales found 539 incidents between January to mid April 2007, compared to 317 in the same period in 2006.
After one incident at a Rhondda station, a train was disabled by a large gang of youths.
The rail company has threatened to withdraw some services.
The incident at Ystrad Rhondda at the end of March left the driver, guard, and three security guards terrified.
Passengers were effectively stranded for more than an hour.
The guard, who had a fire extinguisher let off in his face and a bottle thrown at him, narrowly missing his head, has been off work since the incident.
The guard, Mike Shepherd, said: "I've been on the railway now 28 years and I've never seen it this bad. It's just getting out of control."
Problems are at their worst between Pontypridd and Treherbert
It is still being investigated by British Transport Police.
The increase in incidents, which cover anti-social behaviour, assault, criminal damage, public disorder and attacks on trains, comes despite increased security measures over the past 15 months.
These include security guards on some trains, CCTV cameras and community support officers to boost British Transport Police numbers.
Arriva Trains Wales has warned that unless things improve, it may have to cancel some services on the stretch of track between Pontypridd and Treherbert.
Commercial director Mike Bagshaw told the programme: "Clearly, we're very concerned, this was a very frightening incident.
"Our staff have a right to feel safe in their place of work, and our job as a company is to ensure they have a safe place to work so yes, it's a very serious problem."
Eye On Wales is on BBC Radio Wales on Monday at 1830 BST.