Two youths are being sought by British Transport Police investigating the attempted derailment of a train near Caerphilly, south Wales.
Police say the incident was "extremely serious"
The train's rear coach momentarily left the track in the incident close to Llanbradach station and 60 passengers had to be led to safety.
MP Wayne David has condemned the vandals and a reward is being offered for information leading to an arrest.
Police have been viewing CCTV footage as part of their investigation.
The incident happened at 2105 GMT on Friday when the Arriva train service from Penarth to Rhymney struck a large metal grease pot placed on the tracks close to Llanbradach station.
Sgt James Hartson who helped lead passengers off the train and onto coaches, said the rear coach of the two-coach train actually left the track.
Fortunately, he said the weight of the train carried the coach back on to the track.
"If it was on a bend, it would have toppled over and people could have been killed. Luckily, it was on a straight bit of track," said Sgt Hartson.
He added: "A total of five obstructions were placed on the railway line, one of which was a substantial obstruction which cause the train to leave the track."
The objects placed on the line ranged from boulders and tree branches to metal containers and it was a metal grease pot measuring 3ft by 18 ins (0.91m by 0.45m) which caused the crash, he said.
MP Mr David also said it was clear people could have been killed: "We were lucky this time, but next time we might not be."
The MP, who has been visiting residents in the area, urged police to spare no effort to catch those responsible for the vandalism.
"I just find it incredible," he said.
"The area, like many other areas, unfortunately is plagued by vandalism but this is going way beyond just vandalism.
"We could have had a derailment of a train, we could have had a number of people being killed, and how on earth anybody could engage in such activities is beyond me.
"Those individuals are clearly dangerous to society and they must be stopped immediately and the police must make every effort they can to ensure that they identify and stop those responsible."
Arriva Train Wales described those responsible as putting lives at risk.
A spokesman for the operator added: "Any attempt to derail a train is extremely serious and those responsible have disregard for property and life.
"They obviously didn't consider seriously the consequences of their actions.
"It's difficult to understand and comprehend the actions of these mindless idiots."
Anyone with any information relating to the incident is urged to call BTP on 0800 405040.