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Last week, there was a fatal rail crash. And transport journalist Christian Wolmar made a lot of money. It's a paradox that disturbs Wolmar himself. "There is something slightly weird about earning a lot of money out of a disaster," he says. But when disaster strikes, Wolmar's phone rings and the ISDN he has bought for his study repays its investment several times over.

Last week, there was a fatal rail crash. And transport journalist Christian Wolmar made a lot of money. It's a paradox that disturbs Wolmar himself. "There is something slightly weird about earning a lot of money out of a disaster," he says. But when disaster strikes, Wolmar's phone rings and the ISDN he has bought for his study repays its investment several times over.

He heard about last week's rail crash at 1pm last Tuesday. He then worked till 2.30 the next morning, got up five hours later and worked straight through till 11pm. By the end of the week, he had done 47 articles or interviews for, among others, the BBC, ITN, The Independent, the London Evening Standard, not forgetting The Jimmy Young Show. Last Thursday night was Wolmar night on television, with his views on rail safety being given to Tonight with Trevor McDonald, Channel 4 News, the BBC's 10 o'clock news and to Newsnight.

The former Independent and Observer staffer is one of that small number of specialists who are in every news and features editor's contact book. And when a big story in their specialism breaks, their names are everywhere. When there's an air crash, David Learmount, safety editor of Flight International, clears the decks. His 53 entries on the newspaper library database cover everything from this year's Concorde crash to the crash involving jockey Frankie Dettori to "Would You Take A Flight on Millennium Night?" to assorted hijacks. Over the years though, Learmount's title seems to have changed from Operations and Safety Editor to simply Safety Editor - no doubt easier for the busy commissioning editor to comprehend.

For Wolmar and Learmount, a big payday usually means a crash. At the other end of the spectrum, for style guru Peter York, a big payday means something has stirred in the world of advertising or design. Be it floppy hats or floppy disc packaging, York will be asked to contribute.

Each specialism has its own expert ready to be quoted, write an article or be interviewed on the television news in their 15 minutes of newsworthiness a month. John McVicar has cornered the market in ex-con turned newspaper crime pundit, unless the crime involves firearms, when the phone will ring at Guns Review for firearms specialist Colin Greenwood. The commissioning editor will need to weigh up carefully the size of the bang involved. Small firearms and it's Greenwood; if there's the possibility of a bomb, send for explosives writer Lt Col George Styles.

Some specialisms see their own private wars for media supremacy. Raj Persaud and Oliver James could consult with each other about the psychological traumas of being a leading writer about psychological traumas. Intelligence has its own network of Phillip Knightley, Nigel West and Christopher Andrew, in shadowy competition for the commissioning editor's attention.

Royal wealth, civil liberties, the SAS, they all have their articulate and literary experts in Phillip Hall, John Wadham and Tony Geraghty. It tends to be the more technical specialisms that breed the ubiquitous experts. But even with something like theatre, which boasts numerous staff writers, not least in the Sunday broadsheets with their arts and culture sections, there is nothing that pleases the commissioning editor more than a specialist magazine writer to give readers that sense of objective expertise. So two Sundays ago, Matt Wolf, London theatre critic of Variety, managed pieces on London theatre in both The Sunday Times and The Observer.

The doyen of ubiquitous experts has to be Cary Cooper, professor of organisational psychology and health at the University of Manchester Institute of Science and Technology and unquestionably the most quoted expert in the British media on the subject of work-related stress. His ubiquity has so impressed the senior common room that he has been given the job as Umist pro-vice-chancellor in charge of external relations in addition to his academic duties.

On one summer's day, Professor Cooper could be read casting scepticism on employers' provision of alternative health perks in The Independent, examining workplace bullying for the Evening Standard, and attending a conference organised by Superwoman author Shirley Conran for The Times.

But Professor Cooper never minds a call from a journalist: "As someone interested in stress at work, I like talking to journalists. They often say they are frightened of their editors if they don't deliver the story, and have problems in working long hours and not being able to be with their families."

The logistics for the ubiquitous experts are at least getting easier. If there's a crash, Christian Wolmar will base himself for part of the day at Millbank, where the BBC, ITN and Sky can all interview him at their Westminster studio. Wolmar says he has no difficulty in adapting to the different media and scuttling up- and downmarket, and does not change his style, though viewers might have noticed his syntax being rather more straightforward on GMTV last week than it was later that day on Channel 4 News.

The ultimate accolade, perhaps the key objective for the band of ubiquitous specialists, is to become a Roy Hattersley - an instant expert on everything from constitutional change to Coronation Street, from the third way to Sheffield Wednesday's midfield. With 446 bylines in the last eight years across every sector of the market, Lord Hattersley's income as a journalist must dwarf any salary he earned in politics.

None of the ubiquitous experts wants to spell out their earnings; but Christian Wolmar certainly agrees that his earnings as a much-called-upon freelance specialist are higher than they ever were as a national-paper specialist staff writer. The most prolific, ubiquitous experts will get a good £55,000 to £65,000 a year from their print and broadcasting efforts. Not quite a Hattersley income perhaps, but not bad for never having to suffer the indignity of coming to work in an actual newspaper office.

And, they're an eclectic crowd, the new Hattersleys. On the BBC PM programme last Thursday, Christian Wolmar talking live about rail safety was followed by a pre-recorded interview with author Christian Wolmar about his new book on child abuse. Listening to the car radio, his little daughter asked her mum: "What's Daddy talking about next?"