Vandals have sprayed graffiti over trains on the world-famous West Highland Line in what is believed to be the first such incident on the route.
The attacks happened at the end of the line in Mallaig over three nights this month while the carriages were parked at the station.
Damage included a pink, green and blue graffiti “tag” which covered a large part of one of the coaches.
The Friends of the West Highland Lines said the attacks could put extra pressure on the often crowded line from Glasgow and called for better CCTV coverage to deter the vandals.
Chairman Doug Carmichael told The Scotsman: “Trains stabled at the station’s island platform have been covered in graffiti during the night.
“It is thought to be a first for this very scenic line. Apparently, CCTV cameras do not cover the far side of the platform where the vandals struck.
“It’s quite remarkable graffiti seen in trains in large cities has reached the far west.
“Mindless acts of vandalism like this mean coaches are not now available for the peak summer services on the greatest scenic railway in Europe.
“Let’s hope the culprits are caught quickly and ScotRail erects more CCTV cameras, operational at even the small stations.”
ScotRail said it was “very unusual” for trains on the line to be vandalised.
They have to be taken out of service to be cleaned.
A spokesman said: “Vandalism and anti-social behaviour is completely unacceptable.
“We will work with the British Transport Police (BTP) to bring those responsible to justice.”
A BTP spokeswoman said: “We are investigating three incidents of criminal damage to trains stabled in sidings near Mallaig station.
“All the incidents occurred in the early hours, when carriages were sprayed with graffiti, on Sunday 9 August, Monday, 20 August and Tuesday 21 August.
“Officers are increasing patrols in the area and are appealing for anyone with information to come forward.”
Inspector Bryan O’Neill added: “Graffiti is not a victimless crime and the costs are substantial for the railway in terms of repairs and clean-up.
“Criminal damage of any kind will not be tolerated on the rail network by BTP.
“If you have any information about the incidents or know who is responsible please contact BTP on 0800 405040 or text 61016 quoting ref 75 of 21 August.”