A ScotRail service running between Carnoustie and Aberdeen was approaching Montrose last Tuesday when someone aimed a laser pen at the driver.
The driver had to momentarily shield his eyes, putting himself and the passengers at risk.
RMT’s regional organiser for Scotland, Gordon Martin, condemned the incident which police are now investigating.
He said: “I saw the incident on the latest British Transport Police bulletin and this is very serious.
“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.”
It is not yet known from where the laser was shone but British Transport Police are appealing for any witnesses to come forward with information.
Temporary Inspector Willie Cruickshanks 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 40 50 40 or by texting 61016.
Information can also be given to the Crimestoppers on 0800 555 111.