A train driver has narrowly missed serious injury after a concrete boulder was thrown at the train he was driving.
The large block of concrete was thrown from Cuckoo Lane road bridge, near Doncaster, directly into the path of the speeding train.
The Northern service, which was approaching Hatfield and Stainforth station, was struck on the opposite side from where the driver was sitting, narrowly avoiding "potentially fatal injuries" to the driver, British Transport Police said.
Investigating officer Detective Constable Lee Parsons said: "It is astonishing that someone thought it was a clever idea to throw this very large rock over into the path of this train.
"Trains travel at very high speeds, and a large rock like this can easily break through the glass. What might seem like a bit of fun can have extremely serious consequences."
He added: "I would urge anyone in the area at the time who witnessed people running away from the scene or has any information in relation to the incident to get in touch as a matter of urgency."
Anyone with information about the incident - just after 7pm on Tuesday July 11 - is asked to send a text to 61016 or call 0800 40 50 40 quoting reference 487 of 11/07/2017 or to ring Crimestoppers anonymously on 0800 555 111.