"Go on, check it out," said the ticket inspector. "Just look out the window and watch how everybody smiles as we go by." There's certainly not a lot to be cheerful about on this grey autumn morning, soured by headlines about floods, anthrax and bombing. Yet he's right. There are people smiling. Waving and smiling.
Commuters squashed into a train outside Clapham have their noses pressed against windows. A chador-wearing woman on the Peckham platform lifts a baby on to her shoulders to see. As we cross the bridge over Brixton High Road, an elderly West Indian salutes. The object of everyone's attention is Merchant Navy class No 35005, all Brunswick green and gleaming brass, spouting a towering plume of smoke.
Back in coach B, we're smiling too. Canadian Pacific, 50 years old and one of the most powerful preserved locomotives on the main line, is taking us off for the day to Canterbury, and there's a decidedly naughty feeling about heading out of London on a Wednesday when everyone else is at work. We've hardly got past Catford and we're already into the buck's fizz and smoked salmon.
Yet this is no exclusive trip for good-lifers and high-rollers. The "Cathedrals Express" has been steaming out of Victoria, Waterloo and Liverpool Street every Wednesday and Thursday since July, bound for the great cathedral cities of Canterbury, Winchester, Salisbury, Chichester, Bath and Ely. Most weeks you can just roll up and buy a ticket.
It's one of those thoroughly delightful bits of dottiness at which the British excel. The brainchild of Marcus Robertson, son of Elisabeth Beresford, creator of the Wombles, it sprang from a mix of nostalgia and smart business thinking. Robertson sensed that people were bored with the blandness of modern rail travel and yearned for an old-style glamour. So he hired a loco and coaches from a private railway, and took advantage of one of the few benefits of rail privatisation, which is that anyone has the right to run their trains on the Railtrack main line. This year Robertson bought his own engine, Canadian Pacific, and doubled the number of trips.
Today more than 460 people are piling off the train at Canterbury West station to spend an afternoon exploring the city. Most head for the cathedral, but I browse the second-hand bookshops (picking up a Victorian translation of the Koran), ending up at St Martin's, England's oldest parish church, where St Augustine once worshipped. In the gathering dusk, there are fewer more evocative places in the land.
It's almost completely dark as we puff out of Canterbury, but there are still treats to come. Like the boat trains of old, we run under the white cliffs of Dover, with a watery moon casting a faint glow over the Channel. Then it's back to London, racing the Eurostars, but with the culinary advantage to us, I think, as we dine on roast stuffed guinea fowl with port wine.
The cathedral authorities have been delighted with the train that has brought them so many extra visitors over the summer, so this week and next, Canterbury, Ely and Salisbury are laying on special carol concerts to coincide with the train's arrival. Can there be anything more potently English than a combination of choral music, Christmas, steam trains and cathedrals?
The carol concert trains run to Canterbury on 4 December, Ely on 5 December, Salisbury on 11 December and Winchester (where the concert will be in the Great Hall of the College) on 13 December. Prices per person are £45 standard return, £117 first class with a food hamper there and back, and £147 "premier dining", with champagne brunch on the way out and a gourmet three- course meal on the way home. Concerts are included in the price. Bookings (01483 209888 or 07776 145533; www.steamdreams.co.uk).