I’m surrounded by an extraordinary collection of limestone peaks, rising from the emerald waters of the Gulf of Tonkin, like giant cathedrals. The last time I was here, it was on a Vietnamese navy ship, searching for refugees trying to escape the tyranny of the communist regime.

That was 40 years ago, when I was making a TV documentary, and the beauty of Halong Bay has haunted me ever since. Now the navy is long gone, replaced by flotillas of cruise boats, bringing tourists to marvel at these rocky icons, eroded by the wind and waves, and topped with greenery. I was anxious that tourism may have damaged this lovely idyll, but once the boats are at anchor, it seems much as I remembered it.

A florist in a market in Hanoi
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I’d flown into Hanoi and, as the plane descended through the early-morning mist, I thought of the American bombers flying sortie after sortie over North Vietnam. On my first visit, I’d found a crumbling French colonial city; it still retains much of that charm, although of course modernity has left its mark here, too.

The Old Quarter remains relatively unchanged, though scooters have replaced the massed ranks of bicycles. And what of Ho Chi Minh himself, still lying embalmed in his mausoleum, guarded round the clock? He seems well preserved — but then I gather he’s sent to Russia for maintenance.

But I am not here simply to relive the past and revisit old haunts. My main purpose is to experience the 1,600km journey by train from Hanoi to Ho Chi Minh City or the city I will always think of as Saigon. The single track line that runs the length of the country was built when this part of the world was known as French Indochina, but after it split into North and South Vietnam, continuous service ceased in 1954.

Heavy bombing during the Vietnam War put much of the track out of action but, after the fall of Saigon, the line was restored. Trains started running again in the late Seventies and unofficially services have been known ever since as the Reunification Express.

I board the overnight sleeper and the brand new rolling stock impresses. Second class is crammed with families, getting ready to bed down for the night. I have a first class couchette to myself. I enjoy a decent night’s sleep before arriving in Hue — almost half way down the country – for breakfast.

This city was the imperial capital of Vietnam, from 1802 until 1945 and there are still tantalising glimpses of the grandeur of this time. Although there is clear evidence of wartime damage it still boasts an immense Imperial Citadel on the north bank of the Perfume River, surrounded by six miles of walls, pierced by 10 gateways and punctuated by myriad temples.

Da Nang railway station
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This journey between Hue and Da Nang further south is one of the most exhilarating stretches of the line. It climbs to the Pass of the Ocean Clouds through a series of tunnels and reaches the geological divide between the former North and South Vietnam. Beaches lie below, with hazy islands in front and misty mountains on the horizon. Paul Theroux, in The Great Railway Bazaar, called it one of the loveliest places in the world — and it is obvious why.

Da Nang is one of Vietnam’s largest cities, but we pass without stopping as inside the carriage cheery railway staff dispense mountains of rice and grilled meats from trolleys. I take in the landscape — miles and miles of rice paddies, farmers in straw conical hats.

After all that, it is a shock to arrive in Nha Trang, Vietnam’s answer to Benidorm with its clusters of hotels lining the beach. The Vietnamese are earnest holiday makers and dawn sees the shallows packed with paddlers and day-trippers here to enjoy excursions, snorkelling and mud baths.

Cham towers, Nha Trang
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I forgo the pleasures of the beach for another temple — the eighth-century Po Nagar temple complex. On a low hill just outside town, it was built by the Cham people who once ruled this region. Originally there were several towers, but only four remain with the highest rising to 80ft.

It is topped with a terraced pyramidal roof and inside the vaulted main chamber there’s a huge black stone statue of the 10-armed deity Uma. The Hindu temple has been adopted by Buddhists and I’m surrounded by devout worshippers.

The eighth-century Po Nagar temple complex
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The last leg of the journey sees me back on the rails and it’s getting dark by the time we reach Saigon. Just outside the station looking up, I see a skyline that could easily belong to a booming Asian city. Further into town, I’m pleased to see the Central Post Office and the pink-brick Notre Dame Cathedral have survived.

If you’re interested in the recent past then the War Remnants museum has three floors to tell the grim story of the conflict. The former Presidential Palace has been left as it was when the North Vietnamese tanks smashed through the gates and those same tanks still stand guard.

A trip out of town takes me to the Cu Chi underground tunnels where Viet Cong soldiers hid before launching their final offensive on the city. A section of tunnel has been widened so that Westerners can fit in, but it’s still a claustrophobic experience.

Back in town in the rooftop bar of the Majestic Hotel, I watch the sun go down. The lady behind the bar is intrigued to know that I was in here 40 years ago and asks me what has changed. She listens to my reply and says I’ve missed something important. ‘Now’, she says, ‘people always smile and you’d never see that before’.

The Daily Telegraph