FIRST it's the smell: a heady cocktail of coal, steam and oil. Next comes the high-pitched whistle, releasing steam from the engine.

As I board the train in Caernarfon, the medieval castle where Prince Charles was invested as Prince of Wales towering above me, I know this is going to be no ordinary journey.

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When the Welsh Highland Railway completes its first full 25-mile circuit from Caernarfon to Porthmadog next week, it will mark the culmination of 14 years of hard work to bring the railway back from the dead.

From April 20, it will connect with the Ffestiniog Railway to claim its crown as the longest heritage track in Britain: 40 steam-powered miles through the rural heart of Snowdonia.

I have been invited to an exclusive preview of the newly completed section of the Welsh Highland line to see how the £28million project will open up Snowdonia National Park to a new generation of walkers and wildlife-lovers.

I settle into an armchair in the plush Pullman carriage as we roll into a landscape seemingly composed of brushstrokes by Welsh painter Kyffin Williams.

The landscape bears witness to the tumultuous local history with a mix of quarrying and sheep-farming pasture. Among the industrial heritage, however, yellow-flowering gorse, gurgling streams and isolated cottages speckle the mountainous slate runs with vibrant vital signs.

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As Mount Snowdon, the highest peak in Wales at 3,560ft, looms into view, ospreys circle overhead. Daron Lodge, the train services manager in shiny peaked cap, busies himself with serving up coffee and nuggets of local history for passengers.

"I'm not someone who obsesses over old railways but this one really stands out for the way it highlights the natural beauty of the area, " he says.

Daron, who works eight months each year as one of the railway's 1,000 volunteer staff, enthuses that it's "the greenest way to get around". The route is designed as a hop-on, hop-off service, with regular stops at trailheads, such as Snowdon Ranger station where you can pick up one of the walking trails to the summit of Snowdon.
Services pass by roughly every two hours in high season, so you can head off for a short walk or a pub lunch and catch the next train for another chug through rugged panoramas.

As the train snakes round the switchback S-bends I break my journey. The picture-postcard village of Beddgelert offers a clutch of B&Bs, cafés and pubs around a wild flower-strewn bridge and a riverside walk to a sculpture of Prince Llywelyn's faithful dog, commemorating a favourite Welsh folk legend.

Spring is stirring with the first gambolling lambs and bluebells.

I celebrate my stroll by tucking into two scoops of world-class ice cream from Glaslyn Ices - wild cherry and Turkish Delight. Well, it is the weekend.

Back on the train, the journey builds to a dramatic crescendo as we ride through the Aberglaslyn Pass, named in a recent poll of National Trust members as the most scenic place in Britain. The train edges along the rock shelf above the River Glaslyn, walkers picking their way along a parallel path through the wooded gorge, to assert its claim as the most iconic stretch of the journey.

From here, we descend towards the RSPB Visitor Centre at Pont Croesor with its nesting ospreys. From the restored, open carriage, I can watch the changing of the seasons before my eyes.

Looking around me, the real-life Hornby train set ambiance clearly appeals to men of a certain age with a penchant for mechanical engineering.

However, families, retired couples and a few overseas visitors also join me to enjoy the views. All of them want a photograph with Ben Rose, the diesel engine driver, who maintains the steady 20mph average speed throughout the journey despite regular twists and turns. "It's a challenge.

There's a section from Rhyd Ddu to Hafod y Llyn when the engine is fighting a gradient of 1 in 40, highest in the UK, " says Ben, flashing a white-toothed smile through his grubby driver's facemask. "We'll be shovelling 1.5 tonnes of coal going up that."

We finish, racing across the flatlands on the virgin track towards Porthmadog's Harbour Station, the only section with views across the National Park from the beach. The track may now be complete but the work is not over. The railway is looking to raise £10million for new stations at Caernarfon, Blaenau Ffestiniog and Beddgelert and new coaches.

For me, it is the end of the line. Time to stretch the legs around seagull-central harbour at Porthmadog and sample a plate of home-cooked lamb stew at a pub, washed down with a pint of Purple Moose ale from the local microbrewery.

After a day of national-park landscapes and great Welsh hospitality, I raise a glass to the age of steam.

THE KNOWLEDGE:
Ffestiniog Travel (01766 772030/www.ffestiniogtravel.com) offers packages on the Welsh Highland Railway from £155pp. Price includes two nights B&B at the Celtic Royal Hotel in Caernarfon, return steam-train travel and connecting train tickets from across the UK. Children travel free with a full fare-paying adult.
The Welsh Highland Railway (01766 516000/www.festrail.co.uk) offers return rail fare from Caernarfon to Porthmadog from £32.
Visit Wales: 0870 830 0306/www.visitwales.co.uk