The damage to the Llandudno-Manchester Piccadilly train, the first incident on Saturday night
Three trains are out of service after a spate of vandalism during which metal and bricks were suspended from a bridge to smash the driver's window.
On Saturday evening a train travelling from Llandudno to Manchester Piccadilly was struck. Then the Holyhead to Crewe service was hit by a brick.
Another was damaged on Sunday, but this time a brick or stone was thrown.
British Transport Police said the offences could kill and could be punishable by life imprisonment.
Sergeant Andy Hemmings, of British Transport Police in Bangor, Gwynedd, said: "We're talking ultimately that people could die as a result of these actions.
"In legal terms the offence that we're looking at investigating is what we call an offence endangering the safety of persons on the railway, and that offence with intent carries life imprisonment the same as murder - so very, very serious."
Officers have been collecting forensic evidence and are said to be following other lines of inquiry. As well as plain clothes officers, there are high-visibility patrols in the area.
No-one was hurt in any of the incidents but the trains were removed from service for repair.
Anyone with information is asked to contact BTP on Freefone 0800 405040 or Crimestoppers on 0800 555111.