Shocking clips show thugs targeting trains and walls beside busy motorways.
OLD Firm ultras are waging a vicious vandalism war by wrecking trains and buildings across Scotland with graffiti, we can reveal.
Hardline Celtic and Rangers fans are marking their territory with dangerous paint attacks - then post videos of the damage online.
Shocking clips obtained by the Scottish Sun on Sunday show thugs targeting carriages on tracks and walls beside busy motorways.
Cops warned they are risking lives while train operators revealed the crime spree causes a multi-million pound clean-up operation each year.
British Transport Police (BTP) Superintendent Christopher Shields said: "Graffiti on the railway takes trains out of action, causes thousands of pounds of damage and has a knock-on effect for the whole network.
"It's also incredibly dangerous.
"It often involves trespassing onto the railway lines, which can be charged with electricity, or have trains constantly passing though.
"The railway is a CCTV rich environment and BTP work closely with our industry partners to ensure anyone committing criminal damage on the railway is brought before the courts."
The warning comes just days after Celtic ultras group the Green Brigade released a 90-minute film entirely dedicated to graffiti attacks.
It includes the damage to train carriages, cops taunts and a shocking incident where masked thugs targeted the subway.
A disclaimer at the beginning of the flick - Unlimited Colours - claims the footage was "received anonymously".
It said the film was meant for "documentation purposes only" despite obvious tags linking the group.
The films opens with a clip of vandals spray-paint a ScotRail service with the words "victory to the CFC" before it passes through train stations.
Another clip shows men in tricolour balaclavas target a stationary train with "0141" - Glasgow's phone code - beside a "Green Brigade Banned Bhoys" tag.
The film then shows a group of 10 yobs in boiler suits and masks targetting a Glasgow Subway carriage in front of horrified passengers.
The "criminal damage" incident at Kelvinbridge on Halloween last year sparked a cop probe and was condemned by Strathclyde Transport Partnership (SPT) as "particularly upsetting".
Further scenes show incidents on tracks, train stations and depots across the country with graffiti spelling out "Celtic", "Ultras", "Green Brigade" and "ACAB" - an acronym for"All Cops Are B*****s".
Another clip shows a spray-painted cartoon pig with the word "police" on its cap.
Rival ultras group Union Bears have also documented a campaign of graffiti vandalism in their annual season round-up videos.
The Gers group's 24/25 film shows a group of hoods smear the word "Ultras" on a train carriage and "Union Bears" on a wall behind a moving train.
Other scenes show "Rangers" plastered across a bridge, and "UB07" at the side of a busy road.
In another clip, yobs created a massive graffiti of the sectarian slur "No Pope of Rome" beside the M8 motorway - just hours after the death of Pope Francis in April.
The group's 23/24 video includes an entire motorway bridge covered white, blue and red paint and numerous walls covered in "BRS" and "RFC" graffiti
Their previous year's video shows "UB07" graffiti on train services, "Glasgow Rangers Ultras" on a rooftop and hoods covering a building near Ibrox in blue paint.
It's understood both groups have also clashed over prominent spots by covering up each other's graffiti.
Their films also show graffiti attacks on away trips to Germany, Portugal, Greece and Sweden.
Trespassing on railway lines can lead to fines of up to £1,000.
And Network Rail revealed that damage from graffiti costs the taxpayer more than £3.5million each year.
Scotland route crime manager Neil Cook said: "We're committed to keeping the railway safe and welcoming for passengers and take action wherever possible to prevent vandalism such as graffiti.
"Not only is it illegal, but it's also incredibly dangerous and disruptive."
The body is responsible for removing graffiti around Glasgow Central and Edinburgh Waverley.
Other stations are managed by ScotRail, who were unable to provide cleaning costs.
But service delivery director Mark Ilderton said: "Incidents of graffiti take up valuable cleaning time unnecessarily and those committing it put themselves at risk by trespassing on the railway."
An SPT spokesperson said there had been some succesful arrests and prosecutions of perpetrators.
A Police Scotland spokesperson said: "Graffiti adversely affects businesses and the general public and is a blight on our communities.
"Officers work closely with partner agencies in the run up to events associated with this kind of activity.
"In areas where there are causes for concern or intelligence suggests anti-social behaviour is likely, then extra patrols are carried out."