A train on a miniature railway in Kent was derailed after a plank of wood was placed on the tracks.
Passengers helped to lift the engine's wheels back onto the track
Police said the driver on the Romney, Hythe and Dymchurch Light Railway was on a blind bend when he noticed the 12ft plank lying across both tracks.
They said he had no time to stop and the front two wheels came off the tracks when they hit the wood.
Nobody was injured but police said the incident on Sunday afternoon could have resulted in deaths.
No damage was caused to the train and passengers helped the driver get it back on the rails so the journey could resume.
The steam railway is privately-owned and operates one-third scale trains on 15-inch gauge tracks.
It opened in 1927 and runs on a 13 mile route between Hythe and Dungeness near the Kent coast.
The plank was left on the rails at the back of Jewsons in Dymchurch Road, Hythe.
Danny Martin, the railway's general manager, told BBC News Online: "In reality it only derailed on a pair of wheels but such acts are terrible and totally against what we want.
"We would give every support to the police and we would look to prosecute whoever has done this.
A driver on the railway was killed in a collision with a car in 2003
"Nobody was hurt but you could injure someone and obviously we don't want that - people come here for a fun day out."
Mr Martin said fencing near the scene of the incident had been cut recently, presumably by people trespassing on the line.
Pc Andy Perham said: "It is fortunate that no-one was injured as this could have been very serious and we could have been looking at manslaughter charges."
Kent Police want anyone with information about the incident, which happened at about 1330 GMT on Sunday, to contact them or Crimestoppers, who can be called anonymously.
In August 2003 a driver on the railway, 31-year-old Kevin Crouch, was killed when his train collided with a car on a level crossing.
The car driver, 23-year-old Marie Scrace, from Lydd, was cleared of causing death by dangerous driving but was banned from the road for a year after admitting careless driving.