The Barbican can pack in 2,000 people for a concert. Almost 3,000 classical- music fans can squeeze into the Royal Festival Hall. And Brixton Underground station? 30,000 pass through it every day - and all get a loud blast of the classical stuff whether they like it or not.

Quietly, steadily and, if not secretly, then certainly stealthily, London Underground is rolling out a compulsory classical diet. And it's joining a growing group of local authorities, transport companies and even supermarkets across the country. The idea? If we are all stressed out, we need calming down. And if we are antisocial yobs looking to cause some bother and steal Travelcards, we need moving on. Somehow classical music seems to fit the bill in both cases.

Perhaps this is why Brixton is already well used to it, as I discover while the blast of Schubert's Unfinished is throbbing through the ticket office on a Tuesday lunchtime. The station first got plugged in more than four and a half years ago, a test site to see whether the embryonic scheme deserved expansion. Clearly it seemed to do the job; as of the beginning of this year 40 stations have now been equipped with the necessary kit, and they range from the positively genteel (West Brompton) to the Wild East of the District Line - Dagenham, Upton Park - alongside more mixed South London spots such as Balham and Morden.

Perhaps this tuneful invasion really is as unremarkable as Transport for London protests. “It wasn't really a big policy, but we rolled it out when it was expedient to do so,” says Richard Parry, TFL's director of strategy and service development. He's armed with statistics from satisfied punters, who, he says, feel happier. Less stressed. And, yes, calmer. “Our research says that 80 per cent plus say it makes them feel more relaxed, and 85 per cent plus that the music improves the general environment of the station.” The research Parry refers to was conducted by TFL in 2006, when commuters were questioned at five stations across the network.

Somewhere along the line, however, the official narrative changed. When the first Underground station (Elm Park, on the District Line) got hooked up to classical, this was a story about crowd control. “We can't claim it was an original idea,” Parry concedes. “We knew it was tried in different environments. One we looked at was the Tyne and Wear metro.” The back story there, widely reported at the time, was of vandalism down, problem teens pushed off to bother someone else.

Brixton is certainly an unthreatening place, despite its reputation. “There's so much crime around and I think it's receded,” says a local resident, Lisa Martin. “People aren't really hanging around as much as they used to, 'cos classical music's not really their vibe, is it? It's a psychological thing, isn't it?”

Well, perhaps. With the scheme now being extended to middle-class oases such as West Hampstead, the official line now from TFL is that this is nothing to do with crime or psychological pressure and everything to do with “environment”, “ambience” and “care”. It's also about another C-word - control.

“We want to give people a greater feeling that someone is in control, making things secure and safe,” Parry says. “It's certainly not there as a big deterrent.” Hence station selection is not based on levels of crime, but simply limited to those sites that are not too central (too many people, too much confusion) and do not have an interchange (for the same reason). Parry also confirms that the music is played only in ticket halls rather than in trains (where we couldn't escape) or platforms (where we need to listen for important information). It's the same argument that he uses to avoid the accusation of creating a Big-Brother-style network where our eardrums are monopolised by a loudspeaker.

But if it's trying to show control, it's slightly surprising to find out that TFL doesn't actually have anything to do with the mechanics of the operation at all. When it co-opted the system that the Tyneside metro was using, it asked Metronet, then in charge of the District Line, to find a subcontractor who could supply the technology. What it came up with, installed and compiled by an agency called Broadchart, now plays itself; the only control that station staff have is how loudly they play it.

“People are always saying: ‘Who chose the music?'” grins Mervin Russell, the station supervisor at Brixton, “and we have to say: ‘It's not us, it just comes on automatically at 4.30 in the morning.'” At midnight, apparently, it switches off. Faintly spooky? I thought so, if only because Metronet is now in administration, but its commissioned iMix still pipes through its stations like a creepy memento mori.

Surprise number two: the 40-hour tracklist. OK, the use of rights-free recordings means no Modernists - still in copyright - so no Ligeti, no Stockhausen, no Adès. But hang around long enough and you'll find an obscure Rimsky-Korsakov suite, Antar, the Meditation from Berlioz's La Mort de Cléopâtre, and a slice of Haydn's practically unknown opera, La fedelta premiata. I think you'd struggle to hear any of these on Classic FM even if you stayed tuned in for 400 hours.

But the naysayers have a point. What does it matter what's playing when the intention is just to make a relaxing background vibe? “It's nice for music to transport, to take you somewhere else,” one middle-aged male commuter tells me, ignoring the apocalyptic fury of Verdi's Requiem thundering around him. The other leitmotif from my straw poll is that if classical music does make anyone think beyond a fuzzy sensation, then it's a vague association with film music. “I find it cinematic rather than calming,” says James Khoury, who says he's “getting into classical music”, but, no, he wouldn't try to buy any of the music he heard on the Underground “because I wouldn't have a clue what it was”.

One stubbly thirtysomething in jeans and an overcoat bucks the trend. He knows what's playing - a movement from Beethoven's Seventh - and he doesn't think that this explosive masterpiece is the auditory equivalent of Temazepam. A pity, then, that this is Tom Service, classical-music critic for The Guardian, whom I bump into by coincidence on his way home. “What this reveals to us is the tragedy of listening,” he says. “We are now so inured to classical-music effects that we can't hear this as fiery music, we simply hear the effect of the classical brand.”

That's music to the ears of Graham Sheffield, artistic director of the Barbican, chairman of the Royal Philharmonic Society and now spearheading Hear Here, a campaign to encourage better listening to the sounds around us. “It's real music that one wants to hear, not aural pap. It may have an instant impact but after a while people will cease to notice it's there.”

They're both right, in a sense. Isn't it degrading to the composers flung into this jumble to simply be labelled “classical music”, prescribed in a big enough, noisy enough dose to make us all worry less about signal failures? But, again, perhaps functional music does have a valid place, particularly when its audience seems so mollified by it.

Listening in Brixton station to Wagner's overture to Rienzi - a rarity that somehow crept on to Metronet's iPod - I find it hard to sign up to the cynics' camp. As Sheffield points out, there is always an audience who'll give classical their full concentration. “The commonly held view is that people can't listen any more. I was in the Royal Festival Hall listening to Daniel Barenboim. For 25 minutes, in the slow movement of a Beethoven sonata you could feel 2,500 people really listening, not a single cough.” Are those 2,500 more hurt by Berlioz on the Bakerloo than the degree by which the 80 per cent are cheered up by it?

Maybe the two worlds could meet. The Barbican and the RPS could adapt the list of Tube classics, perhaps bringing in the LSO's live recordings to give a local connection. Electronic display boards could tell us more about the music that we were hearing. “It would be nice if they changed the list every so often,” says Russell wistfully, perhaps thinking about the number of times he has heard Schubert's Unfinished, unfinished. Then his eyes flick back to the delays board, and his hand reaches for the volume knob.

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