A passenger was hit in the face by a brick thrown through a fast moving train window in Lincoln, transport police said.
The incident happened between Doddington Road and Boultham level crossing on Skellingthorpe Road at around 3.40pm on March 24.
The brick was thrown at the train. It smashed a window and struck a passenger causing an injury to their lip. The passenger required further treatment but the injury was not serious.
Officers from British Transport Police issued an urgent warning after the incident.
DC Alan Spillett from British Transport Police said: “This was an extremely serious incident and we are in the process of making a number of urgent enquiries.
“Very thankfully, the victim was not seriously injured, however this could have easily been a tragedy.
“The railway is not a playground and I find it shocking that someone thought it was clever to throw an item at a fast moving train. I am today appealing for any witnesses to get in touch as a matter of urgency.”
Anyone with information should contact British Transport Police by sending a text to 61016 or by calling 0800 40 50 40 quoting reference 362 of March 24.
It can also be reported through the independent charity Crimestoppers anonymously on 0800 555 111.