It took me a day and a half of searching before I found any trainspotters in London. They were standing at the far end of Platform 8 at King's Cross. David Burrell carried a camera, Paul Rea a notebook. They were both in their sixties, both down from Manchester and staying in West Hampstead, which they found convenient for Willesden Junction. Our conversation was hampered by several modern railway blights. There was nowhere to sit and there was nowhere to dispose of our cardboard coffee cups; we kept being interrupted by announcements telling us not to lose sight of our personal belongings. Our conversation was a study in melancholia, covering many of the causes of the decline of trainspotting, including the latest, and deadliest, of all - security fears.

Rea tells me that he started trainspotting in the Fifties “by standing on a bit of unadopted track near Newton Heath maintenance depot. We weren't supposed to stand there but nobody minded.” In the early Eighties he had regularly come down to London on spotting trips with the Lancashire Locomotive Society. I ask if it was thriving. “Thriving! We used to fill up a 52-seater coach every time. Now it's a minibus with 12 at the most.”

The three of us are standing next to a typical modern train: a “multiple unit”. It would strike a passer-by as a series of carriages with no locomotive. Rea tells me that it was a Class 365. Not very interesting is it, I suggest?

“Not really,” he concedes, “but in my mind this place is still full of Atlantics [big steam engines]. I still see it the way it was in The Ladykillers.” I ask the two whether they'd had any abuse from the public. “I've had, ‘Get a life, mate',” Burrell says. “I've had that lots of times.” I then ask whether they'd had any trouble from the railway authorities.

Times Archive, 1938: Mallard breaks speed record

While the record was being made tea was being served and the motion was so smooth none was spilled

“Two years ago,” Burrell says, “when I was taking pictures on Manchester station, I was questioned by a station official. Nothing came of it, but it was, you know, close questioning.”

Others have been more inconvenienced by the increased security across the network, and it appears that one unexpected result of the villainy of Osama bin Laden could be the death of trainspotting. It comes down to a question of identity. Who is to say that the three blokes on the end of the platform with their notebooks, cameras, flasks of coffee and Blue Riband biscuits might not be members of al-Qaeda?

Last month's The Railway Magazine reports an “alarming” increase in the number of readers complaining about the heavy-handed policing of stations. It also draws attention to a poster recently published by the British Transport Police urging the public to look out for photographers who seem, in some way, “odd”. “It's not a systematic persecution,” says Chris Milner, deputy editor of the magazine. “You just get these pockets of jobsworths who don't know the guidelines.”

After the London Tube bombings of July 7, 2005, Milner was party to the drawing up of guidelines intended for people wanting to take photographs on railway stations. They are accepted by Network Rail and the British Transport Police, who publish them on their websites. Photographers are expected to report to station staff and say what they're about. They are, of course, to keep away from the platform edge. Given the nannyish mindset of modern railways (which determines that all train fronts and rears must be painted a revolting yellow) it comes as a surprise that railway photographers are asked not to wear high-visibility jackets - this for fear that they will be confused with station staff.

Milner detects an irony in the implicit wariness of railway photographers. “On the day of the London Tube bombings, Sir Ian Blair was asking for people to come forward with any pictures they might have taken.”

Austin Mitchell, a Labour MP and keen amateur photographer, sees another irony: “We are all photographed dozens of times every day on CCTV, so while the Government can photograph us, we can't photograph anything else.” According to Mitchell, who was recently stopped from taking pictures at Leeds station: “Photography is a public right and that should be made absolutely clear.” He has put down an early-day motion about the matter.

In truth, trainspotters have always had their run-ins with the railway authorities. In his book, Forget the Anorak: What Trainspotting Was Really Like, Michael G. Harvey relates his trainspotting adventures of the Fifties. He describes an expedition made in 1957 to the Ebbw Junction depot at Newport: “The visit proved to be quite rewarding, as we noted no fewer than 126 steam locomotives ‘on shed', but before we achieved this we had to avoid the gateman's attention. This we did by cunningly throwing a selection of stones...and while he went out to investigate we crept behind his hut and into the depot!”

Those knockabout days are over. The change is symbolised by the way that what were once casually yet felicitously called Loco Sheds are now “train care depots” - places bounded by bureaucracy, “the compensation culture” and very high fences.

In the first half of the 20th century, access to railway premises could be gained by informal negotiation, and this was founded on mutual respect. Between 1911 and 1950 The Wonder Book of Railways went through 21 editions, and in those days it was the young lad who wasn't interested in trains who was regarded as a bit weird. Trainspotting specifically grew out of “loco spotting”, a term coined by a young publisher called Ian Allan, who in 1942 began publishing The ABC Railway Guides containing lists of locomotive numbers. But gentlemanly “train watchers” had existed since late Victorian times.

Among their number were the first members of The Railway Club, which was founded in 1899 and is, according to The Oxford Companion to British Railway History, the world's oldest society of railway enthusiasts. Today, the club has 85 members, of whom, I believe, I'm the youngest at 45. There is an annual dinner that begins with grace and ends with a toast to the Queen. Regular talks are held in a room at Marylebone station. Recent ones included “The Railways of Cornwall” and a member of the Transport Ticket Society speaking on “I Don't Know Much about Tickets, but I Know What I Like”. (We can be sure that this was entirely false modesty: the speaker in fact knew a great deal about tickets.)

But in recent years the club reports have made agonising reading. One new member might have joined, but two will have died and one resigned. A few weeks ago, members received a special letter: “The executive committee has doubts about the continued validity of the club...” A meeting will be held in October to decide the club's future. Mike Burgess, its honorary secretary, says: “There's this faint hope that someone will come along with a plan - new blood, you know.”

The North Eastern Railway Association, of which I am also a member, recently wrote to me announcing that it was seeking to recruit young members: “Can you suggest any ideas how this might be accomplished?” None came to mind. Britain is not making trainspotters any more, just as it is not making enough engineers to maintain our main lines. Trainusership may be at its highest since the Second World War, but this is largely because of commuting into London. Fewer than half the children who visit the National Railway Museum in York have ever been on a train, let alone spotted any. Let's get this nasty, tyrannical little word out of the way, and acknowledge that trainspotting is not “cool” and that you call somebody one at your peril.

My friend Andrew was recently ejected from a North London suburban station for being on the platform without a ticket. “This halfwit came after me, moving really fast and speaking into his walkie-talkie.” Andrew had gone there to look at The Aberdonian, a high-speed train running out of King's Cross, and when I put it to him that many trainspotters were having similar difficulties, he replies: “I should punch your lights out for calling me that. I am not a trainspotter. I like good transport design. I like the Coronation steam engines; I like European diesels; I like the Woolwich Ferry, the Isle of Wight ferry and the Routemaster bus.”

Note that he did not enthuse about the Class 365. The utter boringness of modern rolling stock is like bromide in the spotter's Thermos. Not only have steam locos disappeared, but so have locomotives per se. In 1958 there were more than 21,000 on Britain's railways; today there are 2,000 and, instead of being called The Flying Scotsman, they are called things such as Good Morning with Richard and Judy. Instead of locomotive-hauled trains we have the multiple units, which are functional - they are worm-like in that they still move when cut in half - but about as aerodynamically exciting as wardrobes. The decline of the locomotives is the main reason why Ian Allan Publishing stopped bringing out its ABC guides 15 years ago.

At King's Cross, as the Class 365 drew away, to the indifference of bystanders, I put it to Burrell and Rea that excessive security might compound the other factors to kill off their hobby entirely. While they were willing to consider any number of gloomy possibilities, they were more bothered by public abuse.

“We had it the other day in Milford Junction,” David Burrell says. “Two blokes in a white van shouting ‘Stupid bastards'.”

The trainspotters of 2008, it seems, are caught between the jihadists and the white van men. A more sinister pincer movement would be hard to imagine.

Death on a Branch Line, a novel by Andrew Martin, is published by Faber & Faber

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