Whisper in the ear of grizzled film buffs, "This is the Night Mail crossing the border!" and they immediately go into a happy, clickety-clack trance and continue: "Bringing the cheque and the postal order/ Letters for the rich, letters for the poor/ The shop on the corner and the girl next door."

In 1936 the GPO's Film Unit 33, under John Grierson, produced a 23-minute documentary dealing with the only subject that rivalled dogs in the country's affection: steam engines. It followed the journey of the night mail and sorting train on its run from London to Scotland. But for the staggering array of talent involved, it could easily have been as worthy and stolid as most of the documentaries on trades that were then in vogue.

There was Grierson, the father of the maturing documentary industry. The director was 24-year-old Harry Watt, who went on to make The Overlanders. Producer and co-editor was Basil Wright, who had directed Song of Ceylon in 1934; while Alberto Cavalcanti, who later directed Went the Day Well, coordinated sound. Benjamin Britten, aged 23, was called in to create the music and a gangling Wystan Hugh Auden ("a clumsylooking creature," Watts recalled, "with red knobby wrists and hands sticking out of a jacket that appeared much too small for him") had the job of matching words to the rhythm of the chuff-chuff.

The result was a documentary where, for significant stretches, image and words were in perfect harmony, and editing flawlessly served the narrative drive, producing documentary filmmaking of an aesthetic level hardly surpassed to this day.

There is no harder act to follow. But poet Tony Harrison, composer Richard Blackford and director David Thomas have just taken that scary journey back over those revered tracks. The work, for ITV1's South Bank Show about Night Mail, is called Crossings - a salute, says Harrison, to Auden's crossing the border. "You also cross in time," he says. "You cross bridges and you cross between individuals."

Harrison prudently emphasises that this is not a remake but a "revisiting" of the old film: "Night Mail belongs quintessentially to the age of steam. It is impossible to simply go with the idea of remaking it. Even as I speak, they are phasing out the old sorting office, now destined for a museum."

There is at least one crucial structural difference between the films. While memory tells us that Auden's locomotive verse ("The chatty, the catty, the boring, adoring/ The cold and official and the heart's outpouring") raced over every point touched by the train, in fact his verse only runs for four of the 23 minutes. Harrison goes the full distance. The verse for the original Night Mail was actually an afterthought. When the rough cut was shown to Grierson, he realised something was missing: the people who received the letters. Auden made the connections sitting at a bare table at the end of a draughty corridor on GPO premises. "Letters of thanks, letters from banks,/ Letters of joy from the girl and boy."

Harrison does not miss the pain and panic a letter can create: "...in one of those sacks/ is a thin little letter with the weight of an axe." This refers to an HIV diagnosis. In fact, he has a more intimate relationship with the material throughout: "I spent a lot of time on recce. It is a kind of creative chaos, but I like the sense of creative serendipity. Sometimes, when I am on the hunt for an image, I find something which makes my hair stand on end."

At one point, the train crosses a salmon leap. He discovered that, in the post office sorting process, there is a point where the letters come rushing along a chute leap to negotiate a turn. This is called the Salmon Leap. "There was another moment which gave me a shiver of discovery," he says. "When the chutes become choked, they press a red button called The Culler." The link with the culled farm animals littering the modern landscape was irresistible. The route Harrison leads us through is Hieronymus Bosch territory rather than Constable: "The modern Night Mail threads through the map/ of mining communities thrown on the scrap/ Collieries culled like Shilbottle, Shotton/ winding gear felled, and workforce forgotten/ along with culled cattle, culled kingdoms of coal."

This is not the first attempt to reinvent Night Mail. Channel 4 had a go in 1986, on the 50th anniversary of the original. In that, an interloping mail plane won admiration, with the train often merely a docile worm below. It burbled with corporate pride ("No connection shall ever be missed") and condescension ("The key to mechanised sorting is post codes"). From this corporate clothesline flew a few rags of not totally disgraceful verse from Blake Morrison.

Auden's work never touched on the harsh realities of life in the 1930s. The 1986 version at least allowed a veteran rail worker to tell how dangerous and dirty the job could be - a snatched mailbag could kill you if you got in its way. And in winter, flying along with the sliding doors open, it was so cold they had to use piles of mailbags to keep their feet warm.

This sleek, moribund affair showed that the highest technology is barren if there is no true creative passion behind it. Human ingenuity is the more fertile resource. Richard Blackford says Harrison "effectively asked me to be a sound designer rather than a composer". He praises the experimental work of Britten, who would record cymbals and play the sound backwards to blend in with the metallic steamhiss of the locomotive.

The equipment in the 1930s was hideously cumbersome. Film stock was slow. There was no question of recording synchronised sound on a moving train. They recreated the clackety-clack by running a model train to and fro over a toy track. They actually risked lives to get the shots of the mail being snatched from the track, and they found a biblical solution to a window's interfering reflections - a carpenter put it out with a hammer.

But perhaps the most ingenious touch was when the commentator raised his voice over the roar of the train (this was quite unnecessary - the studio train sounds were eminently controllable). Any doubts about the authenticity of that chuff-chuff were blown away.

The result of this guile and artifice was a legendary work of art. It took some courage to revisit it.

· The South Bank Show is on ITV1 tomorrow at 10.45pm.