Graffiti artists the DDS Crew paint London tube trains in a preview for a new documentary, Underbelly. Chris Atkins and Tom Oswald

This weekend marked the beginning of the end for the creaking A-class rolling stock of London underground's Metropolitan Line, the oldest trains on the system.

Among the anoraks and spotters marking the passing of these clattering metal anachronisms, an unlikely group of Londoners may well be paying their respects. The most hardcore of the capital's graffiti writers are this week mourning the death of The Big Met, for nearly 30 years their most prized target. The tube line most reminiscent of the New York subway – its tall carriages, smooth spacious side panels, lengthy routes that run overground to right into the centre of the city – has been since the mid-80s the province of only the highest tier of graffiti writer.

Graffiti serves as varying degrees of irritant to most users of the tube. It is ever present in the corners of one's field of vision, apparently motiveless and meaningless beyond its own assertion of presence. There are other users, however, for whom all the scrawls and scribbles, stick letters and bubbles are an ever changing bulletin, a daily highscore board detailing associates and rivals movements and achievements.

I was 12, indestructible and wondering who I was when I first awoke to the adventure of graffiti train writing. It represented a chance to define myself, and as I had always been artistically inclined, it looked like something I could be good at. Once immersed in the scene, the lure and the lore of the tube proved hard to resist. All other achievements in graffiti were walks in the park by comparison. "Real writers paint tubes", I learned. There were photos swapped, cat and mouse stories told, legends created. A London graffiti vernacular sprang up, lines and depots gained nicknames.

In time, through an old friend and veteran tube writer, I was introduced to the tunnels and underground layups. My first time underground brought me to a new world. A space that lay beneath 7 million people, not built for humans, rarely seen and even more rarely explored. To get to the trains involved ducking cameras, traversing catwalks, climbing down poles. Writers hid in alcoves as conversing cleaners and security guards walked past inches away. Wading through layers of black powder, eventually the trains were reached, stabled in a depot deep under central London. Four hours went by in a series of pounding heartbeats and high adrenalin. That night was my first introduction to the DDS crew, a collective of writers, who over many years, more than any other graffiti artists in Britain, have come to symbolise the London underground graffiti culture.

For nights after that, I saw the trains and tunnels in my sleep. The mix of adventure and rebellion, victory and comradeship was intoxicating. There are many others, like me, who have had that experience, and many who have made it in and out of the train yards and tunnels under the city far more times than I did. There are a few of those who have made it their lives work, and for years have risked their safety and liberty repeating that London underground graffiti experience with a dedication and commitment that borders on obsession.

But in pre-Olympics London 2012 things have changed. LU's current policy of immediately removing painted trains from service means now that artwork never runs, and with ever more intense CCTV surveillance and hi-tech security being used to guard the trains, painting tube trains became a darker, more clandestine pursuit. Security patrols in depots mean those who make it in can only get safe painting time of mere minutes, instead of the hours they once enjoyed. This compromises the quality of what is possible to achieve, say the writers.

I was caught and prosecuted, paid large fines and worked off lengthy community sentences. The scene changed, my life had changed, and I moved into legal and more indelible forms of self-expression. Having stayed in sporadic contact with the graffiti scene, I was approached many years later by what became the protagonists of my documentary about graffiti on the underground, and was amazed to hear not only that the network was still getting regularly hit, but that the tactics used today were unheard of in complexity. Not only this, said my hooded friends, but we've been filming it all. But the more time I spent with them and the footage, the more it seemed clear that the culture of tube graffiti is facing its last days. An aggressive graffiti cleaning campaign is under way, graffiti artists are the subjects of large-scale police operations, and lengthy jail sentences now handed down. For many writers the fun and the romance is gone, replaced with hard slog and homework in exchange for ever dwindling rewards and ever increasing penalties. They speak in frustrated tones about being misunderstood and demonised. Their motivations are pure, they say, they want to paint the best and most beautiful art they can and they appear baffled that the public is so ungrateful.

Because they have experienced it, they don't realise that most Londoners will never know the feeling of seeing one of their carefully painted "window down whole cars" in full colour, with cartoon characters and a message, rolling through their station on a grey Monday morning. And the way things are headed, they may never do.