The unmistakable chime whistle of a Gresley A4 Pacific, which formed part of the soundtrack of my early life, carried above the traffic noise as we strolled towards Oxford station last Thursday morning. I knew at once there had been a change of plan for our Cathedrals Express excursion to Swanage.

Actually, I should say another change of plan. The locomotive scheduled for the trip when we made our booking had been the BR Standard Class 7 Pacific Britannia. Later this was switched to Sir William Stanier’s Princess Elizabeth, an engine only slightly down the pecking order for female celebrity since it was named for The Queen. A first ride, for me, behind this record-breaker had seemed a fitting way to mark the Diamond Jubilee.

Why the change now, to Bittern, the last of the A4s in regular service? It was to be an hour before we had the answer to the question, after we boarded the excursion. This was at Didcot, to which we travelled on a diesel train, the planned Oxford stop for the special having been ruled out by the rail authorities some days earlier.

As we sipped champagne and pondered the enticing prospect of breakfast (croissants, yoghurt with granola, porridge, full English fry-up), we heard over the train intercom the story of Lizzie’s failed ‘health’ inspection the previous day and the last- minute substitution of the A4.

By the most stupendous piece of good luck, Bittern had returned hours before to her London base from a visit to the Midlands. Engineers worked through the night to prepare her for the trip, which began in Banbury, while Network Rail officials did what was necessary to clear her path to the south.

Only an A4, perhaps, might have been considered a worthy substitute for Princess Elizabeth. With their good looks and reputation for speed (Mallard is a member of the class), these locomotives were a legend throughout their days of regular service and — for the happy survivors — in the near half-century since.

Bittern was a familiar sight in my childhood — Brunswick green in those days and without the valances over the wheels — blasting through Peterborough at the head of some crack East Coast express. Once the diesels arrived, it was one of the favoured members of the class given an Indian summer on the three-hour expresses between Glasgow and Aberdeen. These duties ceased in 1966.

Though the class had been designed to power a series of light, high-speed LNER streamlined trains — The Silver Jubilee, The Coronation, The West Riding Limited — the A4s were soon to build a reputation, particularly during the war, for tackling huge loads.

This ability was certainly demonstrated on last Thursday’s trip, when some extremely sprightly running was put up with a very heavy train, within the 75mph maximum allowed by Network Rail.

The only hiccough on the way down to Swanage occurred on the last leg, beyond Wareham. Unable to restart the train after an unscheduled stop, the engine was forced to make a tactical withdrawal for about a mile to ‘take a run’ at the incline, which she then managed with ease. An expert on board (and there are always plenty of those!) told me the problem was not with the engine but the track. It was wet and greasy through not being used regularly, since we were straying off Network Rail metals on to a privately run line to complete the journey.

I wish I had space to describe the excellencies of the Swanage Railway, which has an important function in bringing thousands of visitors to Corfe Castle and Swanage. Go to www.swanagerailway.co.uk to find out more. What I shall do, though, is offer a couple of quotations about our run supplied by Martin Payne, the commercial manager.

A curious aspect of a trip like this is that from the comfort of your coach you miss the thrill of seeing the engine in action. Martin helped fill this gap for me say by saying: “Bittern really took the three-mile climb from Swanage up to Harman’s Cross in her stride and it was hard to believe she was hauling 13 coaches — one of the longest and heaviest trains ever to leave Swanage station with just one locomotive on the front.

“The bark from Bittern’s exhaust was wonderful to see and hear — it’s a magnificent steam locomotive and like a finely oiled sewing machine.

“Bittern is a sleek and powerful machine; the sort of impressive steam locomotive that Agatha Christie’s Belgian detective Hercule Poirot would have travelled behind.”

And I am sure that, like me, he would have travelled Pullman class and enjoyed the delicious dinner that we were served as Bittern laboured. It was, since you ask, canapés, celeriac and apple salad with Cambrian ham, confit of Loch Duart salmon, champagne and strawberry mousse, and cheeses (Cornish Yarg and Shropshire blue).