On the side of a railway bridge over Barrington Road in Brixton, south London, are the last three tags painted by Harrison Scott-Hood, 23, Alberto Fresneda Carrasco, 19, and Jack Gilbert, 23 – “Lover”, “Trip” and “‘Kbag”. They are likely to stay there for some time, as a poignant memorial to the three young men whose bodies were found 350 yards down the track on Monday morning. All three had been hit by a train.

All week, graffiti artists – or “writers”, in the language of the subculture – from around the world having been paying tribute to the three men. An Instagram page has been collating pictures of their graffiti, as well as artistic tributes. At Loughborough Junction station – the stop closest to where the three died – graffiti writers were among the mourners who have contributed to a shrine, with spray cans nestled among bouquets, and messages sprayed on the wall.

But there has also been criticism. A former Transport for London board member, Brian Cooke, who was recently suspended from the Conservative party, tweeted they were “common scum and criminals”. It was met with a furious response. But judging by the comment sections of popular news websites, he was by no means alone in his opinion. The tragedy has prompted a fresh debate over the artistic merit of writing on walls.

Those who travel on trains in a city will be familiar with the graffiti that lines the tracks, executed with varying degrees of skill and creativity. Figures from British Transport Police show “graffiti-related crime” is on the rise across Britain, with 2,498 reported incidents in 2017, up from 1,413 in 2013. That rise is reflected in London, where there were 662 reported incidents in 2017, up 86% on 2013. The first quarter of 2018 had the highest number of reported graffiti crimes in London since 2013.

Jack Gilbert, 23 (left), Alberto Fresneda Carrasco, 19 (centre), and Harrison Scott-Hood, 23, who were killed on a train line in south London. Photograph: British Transport Police/PA

But the statistics are limited. Graffiti is so ubiquitous on urban train lines that most goes unreported; it would be an impossible task for BTP officers to conduct audits to estimate its true scale. Network Rail said it spent about £3.5m a year removing graffiti, a figure that has not changed significantly in recent years but it did register a 28% rise in “trespass-related incidents” between 2014-15 and 2016-17.

Deaths while walking the tracks are relatively rare, according to Joe Epstein, who runs the website LDNGraffiti. That is despite writers gaining and giving respect not simply for the technical skill and creativity of their pieces, but also the inaccessibility of the spot and the risks taken to reach it.

“In graffiti, the harder the place is to reach the more recognition it will get,” he said. “There’s terms like ‘king spots’; there’s a term – it’s not very appropriate – but it’s called a ‘heaven spot’, which is essentially a location that’s so hard to get to and so risky you could potentially lose your life. When writers hit these spots, they are widely recognised.”

The stretch of line where Lover, Trip and Kbag were struck is famous among graffiti writers in London, one told the Guardian, with some of the city’s oldest illegal works, dating back two decades or more. Elevated to a height of 65ft, the route leads into central London, increasing its visibility, via Brixton, a hub of London street culture. The three had travelled from north London to paint there.

The highest accolade for a graffiti writer is going ‘all-city’, which is having tags on every line, in every postcode Theo Kindynis

“With subcultural graffiti, a lot of people are like: ‘Ah, they’re marking their territory,’” said Theo Kindynis, a criminologist at Goldsmiths, University of London, whose work has focused on the scene. “But it’s never been about that. The highest accolade for a graffiti writer is going ‘all-city’, which is having tags on every line, in every postcode ... So wherever guys are from, they are going to want to come and paint in places where they otherwise wouldn’t usually go so that other writers from those areas see them up and it increases their subcultural cachet.”

As the Mail Online pointed out this week, with its story on Lover’s £7,000-a-term public school education, those involved in graffiti range far from the common conception of teenagers from rough council estates. “They’re young men, but other than that it’s not a very homogeneous scene demographically,” said Kindynis.

“You do get people from very fortunate backgrounds; people dating supermodels and this sort of thing; you get actors who are involved. But you have also got people that are in and out of jail, have had very difficult upbringings. It’s not like the really, really desperate hard-up kids that are doing this stuff because ultimately you’re not profiting from doing this ... It’s not the sort of really fringe parts of society that it’s made out to be, these people often have day jobs and whatever else.”

Authorities are quick to brand graffiti as simple vandalism. But that tension is apparent within the “graff” scene itself, where artists prefer to be known as “writers” and talk about wanting to “claim space” and “do damage”, according to Simon Armstrong, who is currently researching a book on London graffiti. “They don’t really care what you or I might think, that’s less of a priority,” says Armstrong, the book buyer for the Tate galleries’ shops, who has observed the graffiti scene for more than two decades.

Nonetheless, he says, for many young people it can be a route into successful careers. “There’s so many examples of graffiti artists who started out doing trains and now they are doing unbelievably amazing art,” he said. “Many of them are self-taught; not many of them went to art school, or they go later and do design courses.”

The discrepancy between graffiti as both vandalism and art reached a zenith in 2010, when the then prime minister, David Cameron, visited Washington DC and presented Barack Obama with a canvas by Ben Eine. “That’s mad, because Eine was one of the hardest London writers,” Armstrong said. “His tag was everywhere and to be embraced in such a way is really quite peculiar.”

And yet there are still those who insist graffiti has no artistic merit and the authorities take the investigation of graffiti on the railways seriously. According to a spokesman, the BTP regularly runs high-visibility patrols in hot spots to deter writers. This week, six officers were stationed at one point to police the tributes to Lover, Trip and Kbag. It is understood the force has recently reformed its vandalism squad.

British Transport Police officers carry evidence bags as they leave from Brixton train station during the investigation into the deaths at Loughborough Junction. Photograph: Niklas Halle'N/AFP/Getty Images

“In addition to these measures, we regularly meet with the rail industry to discuss preventative steps that can be taken to deter offending,” a spokesman said. “This includes the use of both overt and covert CCTV cameras, highly visible patrols of police and security staff, educating young people about the dangers of trespassing on to the rail network and introducing anti-graffiti technologies.”

On Friday, an inquest at Southwark coroner’s court heard that Lover, Trip and Kbag were identified by their fingerprints. They suffered multiple injuries after being hit by a train just after 1am, and their bodies lay on the tracks for nearly seven hours before they were noticed.

A Network Rail spokesperson said: “The railway is an extremely dangerous environment if not being used correctly. It’s full of obvious and hidden dangers, all of which can be lethal. Please, never walk down on to the tracks of the railway for any reason.”

Friends of the families of the three men who died have set up a GoFundMe campaign to help with the cost of their funerals.