Whoosh... and the beach special steams back to the Thirties

Michael Williams heads to the South Coast on a train where a generous helping of nostalgia is served with the champagne

A little boy with curly hair and short trousers, with a tiny attaché case, looks up, rapt with admiration for a steam engine at the end of a station platform. It could be Waterloo, Charing Cross or Victoria - who knows? What matters is that the sun is shining and he's off for a holiday at the seaside. The driver beams down appropriately from his cab.

We're deep into the itinerary of memory here. The image is one of the classic advertising icons of the 1930s, promoting summer holidays on the old Southern Railway. It is even more potent now, offering an instant nostalgia high. This was an innocent world, before Virgin Trains, the demise of the nuclear family, before the station buffet became Burger King, before the advent of the Game Boy, when children could be happy digging with a tin bucket and spade on unpolluted beaches on golden afternoons that never seemed to end.

At least that is what I am thinking on platform 14 of Clapham Junction station this cloudy early summer morning, jostled by city-bound commuters who are tutting about this odd bunch of people with beach towels and cameras, some with the temerity to have small children in tow.

But no time to day-dream. Here it comes - a few puffs of smoke over the horizon towards Chelsea turn into a screech of brakes, a cloud of sulphur and a hiss of steam as Standard Class 4-6-0 No 73096 pulls in with the Sunny South Special, bound for Bournemouth, Poole and Weymouth. All aboard, for the once ubiquitous but now entirely novel idea of a day excursion by steam train to the seaside.

The best back-to-the-future ideas, like Hovis and Mr Kipling, are often the brainchild of marketing men. Marcus Robertson, the impetus behind the "Sunny South Express" is one such man, having made his pile as a founder, with John Arlott, of one of Britain's leading sports PR agencies. But Robertson has always, spiritually, been the little boy in the poster. Four years ago he sold up and invested in running steam trains on the main line, not to make money (he may well have lost a fortune) but because it was a shame, he says, that the idiosyncratic charm of railway travel was being eliminated by the private operators.

It is hardly surprising that at 48, Robertson is going on 13 since his mother is Elisabeth Beresford, creator of the Wombles, and just over there, on the right, as we slow down to pick up more passengers at Wimbledon, is their famous Common. While he wasn't exactly "wombling free", the young Robertson had his Ian Allan trainspotters' guide in hand and has never quite been able to put it down since.

"For my parents' generation, the train excursion to the seaside was a staple of summer life. But then along came the accountants and the intensive use of rolling stock and it all but disappeared. Then last year I spotted an item in the paper - on a single day, there were 150,000 people on the beach in Bournemouth. I thought we could do more for them than a sweaty, nose-to-tail experience down the M3."

So here we are, speeding through the New Forest with the sun just piercing the clouds. A champagne brunch has put the passengers in top form. The "green machine", as our 76-tonne loco is known, is ticking along at the front at 60mph, like a well-oiled grandfather clock.

Yet Robertson is nervous, head darting in and out of the window, peering at his watch. "I don't want the signalman to pull us over," he says. One of the few advantages of rail privatisation is the principle of "open access", whereby trains like this have, by law, to be accommodated, so long as they are safe and able to pace the modern expresses running on the same tracks. A shadow over today's trip is an incident two months ago when a venerable express engine bent its coupling rod on the London to Glasgow main line, causing Network Rail to give all main line steam operators a yellow card.

So there's relief all round as we steam into Weymouth bang on time for lunch. This is fortunate - since we have picked up the town's regalia-decked mayor in Bournemouth on the way, who clearly recognises that events like this can reawaken resorts that have been slumbering in the shadow of the cheap foreign holiday since the 1950s. We are not disappointed. The town, with its golden crescent beach and newly spruced-up Regency seafront is like something out of Shirley Hughes's 1970s classic Lucy and Tom at the Seaside. We have a full five hours here to paddle, to dig for crabs and to gawp on the harbourside. The draw this afternoon is a Michael Henchard-like figure of a fisherman (this is Hardy country), who wrestles thrashing small sharks from his boat on to the quayside. Back to the station and time to stop for a cone and marvel at the Art Deco of Rossi's seafront café.

The "green machine" doesn't let us down as we speed back through the lengthening shadows of Dorset, Hampshire and Surrey over a freshly prepared supper of fresh crab and prawns, followed by roast guinea fowl. Late commuters blink as they watch 73096 uncouple at Clapham Junction. (Was that a steam train, or a drink too many at a City wine bar?)

As the signals change and she chuffs off into the night, the lights pick up a plaque under the boiler: "Built Derby, 1955". Younger than Mick Jagger, Tony Blair, Paul McCartney. Like the idea of the day excursion to the seaside, pensioned off almost certainly before its time.

The Sunny South Special runs to Weymouth on 3 August, 18 August and 1 September. Prices start at £39.50. The train also runs as The Cathedrals Express to various English cathedral cities most weeks until 21 December. Booking: 01483 209888 or www.steamdreams.co.uk.