Police are appealing for witnesses after a train from Kent to London hit a gas canister thrown on the rail line near Swanley rail station.
British Transport Police detectives say it was by "sheer luck" that the Gillingham to London Victoria train was not derailed.
The incident on Saturday caused damage to the train and tracks, but no one was injured and the train remained upright on the tracks.
Detective Sergeant Tony Samiotis said: "We believe the gas canister was thrown onto the tracks and we are appealing for anyone with information to come forward.
"Fortunately, no one was hurt during the incident, and the train remained upright on the tracks. However, this was just by sheer luck and the consequences could have been far worse."
He added: "I cannot over-emphasise enough the dangers associated with this type of crime, it is vital that we find those responsible. Trains travel at high speeds and there is a very real risk when they strike objects on the track that they could be derailed.
"We are urging anyone who was in or around that area to contact us if they saw anything suspicious or have any information that may help us with our enquiries.
The incident caused over 100 minutes of delays to services through the area.
Anyone with information should contact British Transport Police on 0800 40 50 40, quoting reference B11 of 24/06/13.
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