The driver had to shield his eyes when a laser was shone through the front window of the train on Tuesday 29 September, endangering everyone on board.
The incident occurred when the ScotRail service between Carnoustie and Aberdeen was about 500 metres south of Montrose station on the Rossie Viaduct.
Nobody was injured, but the British Transport Police have launched an investigation and are appealing for witnesses to come forward with information.
Gordon Martin, RMT’s regional organiser for Scotland, outlined the potentially calamitous consequences of the incident.
“I saw the incident on the latest British Transport Police bulletin and this is very serious,” he said.
“It is putting the safety of the passengers and the train crew at risk.
“If the driver was approaching a signal it could have been very dangerous as it could have given the driver a fright and caused him to miss the signal.
“The other danger is that it can cause permanent damage to the driver’s eyes.
“It’s totally irresponsible and I would condemn it entirely.”
Willie Cruickshanks, Temporary Inspector for the British Transport Police, said: “The driver has been very lucky in that he was able to look away from it.”
“Shining laser pens can cause issues where the driver could have been put in a state where he could not take the train forward.
“Lasers can also cause damage to the eyes by burning them.
“We are now appealing to the public to provide any information to help us with our inquiry to stop this from happening again.”
British Transport Police can be contacted on 0800 405040 or by texting 61016, while information can also be given to Crimestoppers on 0800 555111.