Reduced maintenance of tracksides is to blame for the damage to The Royal Scotsman, The Jacobite and other private trains that run tours around Scotland. Critics accuse the track operator of being more willing to pay for carriages to be repainted on a regular basis rather than cutting back vegetation on long stretches of rail line.
But passengers, some of whom have spent about 5,000 for exclusive trips, are complaining that dense stands of trees and bushes on embankments are obscuring the views they have paid for.
The problem is so bad in some areas that twigs and leaves have fallen into compartments through open windows. Total payouts by Network Rail are believed to run to tens of thousand of pounds. Repainting a train such as the nine-carriage Royal Scotsman, which is operated by Orient Express, is understood to cost more than 10,000.
Some of the worst areas for damage include the Edinburgh-Dundee line around Ladybank in Fife, and between Garelochhead and Arrochar on the Glasgow-Mallaig route, which was voted this year as the world's top rail journey by readers of travel magazine Wanderlust.
A source close to The Royal Scotsman said of the tree damage: "It happens every year and is very frustrating. It certainly gives a bad impression to high-spending tourists. Network Rail really needs to spend more money on vegetation control, and pressure needs to be brought to bear."
Ann Glen, a director of Highland Railway Heritage, which promotes the region's lines, said tree damage
was "an appalling problem. Whole carriages have been scraped relentlessly. Some of the vegetation growth is substantial and it may even be affecting the ballast around the tracks.
Network Rail has made some effort (on] clearance, but its priority is to ensure the line ahead and signals and bridges are not obscured."
Glen said stretches of track were little better than "green tunnels". The poor state of line-side maintenance contrasted sharply with the steam era, she said, when vegetation had to be strictly controlled to ensure it was not set on fire by sparks from locomotives.
Roger Haynes, commercial manager of the Scottish Railway Preservation Society's rail tours, said it had faced problems getting repair bills paid by Network Rail because the work was done by volunteers. He said: "The problem has been getting steadily worse, which is dispiriting.
What's the point of having one of the world's most scenic lines if you can't see the views?"
Network Rail said its focus was on safety. "On rural routes, priority is given to tackling vegetation which affects sighting distance or has the potential to impact overhead lines or create a blockage on the line," a spokesman said.
The problem appears to be restricted to trains run by private operators. According to Network Rail, ScotRail trains were less prone to damage as its paint includes a protective layer.