A peak performer: Simon Calder climbs aboard the Welsh Highland Railway

Nature runs wild in Snowdonia National Park, which is its main appeal – unless, like Paul Lewin, you are trying to run a railway. "Believe me, in 50 years nature can do quite a lot of damage to a railway line. We've had to rebuild the fences and the drains, remake all the bridges and clean up all the tunnels." And all so that day-trippers can enjoy the wild side of Snowdonia.

We talked as we trundled at a gentle 20 miles an hour through increasingly savage terrain. Mr Lewin is the slim controller of an even thinner railway: the two-foot gauge Welsh Highland Railway. What (you may well wonder if you are not an aficionado of "heritage railways") exactly is that – perhaps a cut-down version of another line in the mountainous Celtic fringe, Scotland's West Highland Line?

Well, there are similarities: both run through some of the finest scenery in the kingdom, thanks to heroic efforts by long-dead engineers. But while the Scottish line is a full-size, paid-up member of the national rail network, the WHR is a scaled-down track to nowhere. But not for much longer. On Wednesday, the penultimate stretch of the Welsh Highland Railway is to open. It starts at the Norman fortress at Caernarfon, and ends tantalisingly close to the Cambrian Coast Line, connecting the far north-west of Wales with the rest of the world (or at least Birmingham International).

The Welsh Highland Line was one of the last gasps of the great railway revolution that infiltrated almost every acre of the kingdom. It rises from sea level to 650ft as it climbs through the heart of Snowdonia, skirting the western slopes of Wales' highest mountain, Snowdon, before descending to the harbourside at Porthmadog. Freshly laid tracks in that small port will allow it to run partly along the main road. But that final link has to wait until next year; right now, Mr Lewin is jubilant about the latest step in embellishing the appeal of north-west Wales.

By rights, the event should have taken place at Easter, since the Welsh Highland Railway is soon to become a resurrection shuttle.

Most heritage railways play a useful, if limited, role for the traveller in Britain. Lovingly preserved by enthusiasts, they provide fascinating glimpses into the days before Network Rail and Standard Class. But all that most deliver to the average day-tripper is an amiable ride in a train hauled by a steam locomotive through pleasant countryside. You tend to go from nowhere to nowhere, and then back again. Personally, I'm more of an A to B man. Which is why the plans to link up with the national network, and open up transport possibilities in a remote part of Wales, is so exciting. And why I joined Mr Lewin aboard the proving trip for the WHR's new, extended service.

We travelled in the closest any heritage railway gets to Club World standards of comfort: with coffee and cakes taken in the observation car that is tacked on to the end of the Third Class carriages. This throwback to the early 20th century was named by the Queen earlier this month. Despite the retro style, it is a brand-new creation that mirrors the carriages that run through the Rockies. The name, Glaslyn, is taken from the river whose valley much of the line follows. As you laze in an armchair, some of the finest vistas in Wales glide past.

"This railway is quite different from any other railway in the country," says Paul Lewin. "We wanted to make it a very different experience. And with the observation car, with the big glass end, the idea was that you could get as close as possible to the scenery of Snowdonia, without actually being in it."

Touches of class are everywhere: a solemn maroon livery so as not to detract from the views, and a lining of cedar, because of the scent it delivers. And talking of scent: on most trains in Britain, you rarely spare a thought for how the thing works, so long as it gets you there on approximately the right day. But steam locomotives are as close as technology can get to a living entity. Man's creation (if that sounds a touch religious, so much the better) of a fire-breathing monster to haul trains is a splendid example of what imagination, engineering and the human spirit can concoct.

This particular locomotive was built in 1930s Belgium for South African Railways; like the line itself, it was abandoned and was rusting away until the Boston Lodge works of the Ffestiniog Railway (also part of Mr Lewin's modest V

C empire) and a benefactor restored it to full, wheezing, whistling health.

I was allowed to climb on to the footplate, where Richard Stagg, the volunteer fireman, sweated as he shovelled a ton-and-a-quarter of Polish coal into the furnace: "This is far more fun than a workout at the gym".

The driver had flown in from Germany to do a job that looks as complicated as conducting an orchestra, with regulators and whistles and gauges to worry over. "This is the perfect antidote to a desk job," said John Bell, who is normally to be found developing software in Heidelberg.

They hardly had a chance to look at the view, so I enjoyed it on their behalf. As you ride through the foothills of Snowdon, the reasons for the commercial failure of the original – and the high hopes for the resurrected railway – flood past the windows. The line does not enjoy a fruitful catchment area. Three miles out of Caernarfon, at Dinas, it turns sharp left and begins to claw its way through terrain where humanity, and therefore custom, is thin on the ground. By mile nine the real drama begins, with the lake of Llyn Cwellyn to the right and the highest mountain in Wales to the left.

Snowdon Ranger sounds like a racehorse, but is in fact a youth hostel – and one of several places from which to begin a walk to the summit of Snowdon; Rhyd Ddu, the next stop, provides an alternative ascent. A rare straight stretch leads to the highest point on the line, which coincides with one of those local geographical curiosities that is either a natural wonder or a tacky tourist attraction: "Pitt's Head", a pile of stones supposedly resembling the profile of William Pitt (the Younger, since you ask).

All downhill from here, which no doubt presents more challenges to Mr Bell at the controls but gives Mr Stagg a bit of a breather. The engineers had to find some way of shedding a lot of height in a confined distance. They settled on what the insiders call "S-bends", but which to me look much more like a diagram of the large intestine. Whichever description you prefer, they swoop and swerve through Beddgelert Forest.

Beddgelert station is a frankly ordinary-looking piece of infrastructure. In order to get the line to pay, the trains have to run with 10 or 12 carriages (which also explains why the engines are so tough). Therefore the platforms had to be extended, in contrast with the pretty little halts that many other heritage railways offer. Indeed, Mr Lewin foresees a time when the line could be electrified: "Although, at the moment, we use steam engines for most of our trains, this railway goes through such stunning scenery that one could imagine, in 50 years' time, that electric trains could be the main source of attraction here."

As we entered the most theatrical stretch of line, the means of haulage became irrelevant. The Aberglaslyn Pass is where road and railway are crushed into a canyon that squeezes between hulking walls of rock. The Glaslyn River races the train, which seems at times to teeter on the edge of a ledge. This pass was rated by the National Trust membership as the most scenic spot in Britain – but you can never appreciate it as much from the A496 as from the Welsh Highland Railway.

The train rumbles out of the view and into a tunnel; at the far end, the terrain has changed once more. You find yourself passing what look, for all the world, like sea cliffs. That is because they were until 1811 when the Cob was built across the Glaslyn estuary by William Maddocks. He managed to drain the estuary, which now comprises flourishing farmland, leaving the cliffs high and dry.

Look on the appropriate Ordnance Survey map and, just north of Porthmadog, you will see a faint dotted line marked "dismtd rly". That is the final stretch of the about-to-be-opened extension. It crosses the river and settles at the newly constructed station of Pont Croesor. A reclaimed railway running on reclaimed land, and furthermore one that ends at an RSPB observatory where you may catch a glimpse of the Glaslyn ospreys.

As we left the train at a station so new that our heels left marks in the Tarmac, I asked Paul Lewin one final question: like every other heritage railway in the country, isn't the WHR just an excuse for boys to play with trains?

"Well, if it wasn't for the boys who like playing with trains, then the railway wouldn't have been built. But it is a major tourist attraction, it employs over 65 people all year round, rising to over 100 in the summer. It supports the local economy to the tune of nearly £15m a year. So, it may have started with boys playing with trains, but now it's vital."

"A home for fallen buildings": that is how Sir Clough Williams-Ellis, creator of Portmeirion, once described the village. His delectable Italianate folly is served by the other half of Paul Lewin's great railway empire; you alight at Minffordd Station, which also happens to be Mr Lewin's home and must be one of the more agreeable tied cottages in Britain. Wales is surely the home for fallen railways – all the more glorious in their resurrection.

Welsh Highland Railway: 01766 516000; welshhighlandrailway.net

Lines of beauty: Three Welsh main-line wonders

Given the sparse population, uncooperative terrain and the UK's neglect of rural transport, Wales has a remarkable density of railways that are part of the scenery – indeed, that hurry you through some of its finest landscapes. Here are a few of the best:

North Wales Coast Line

The only problem with this line is that the Arriva Expresses and Virgin Pendolinos travel too fast as they speed from Chester towards Holyhead. The stretches closest to England give views of the Wirral; the train pauses at a series of seaside resorts; and as you get further west the line reveals glimpses of Snowdonia. Then across the beautiful Britannia Bridge spanning the Menai Strait, fast across Anglesey (pausing at the station abbreviated to "Llanfair PG" and one final water crossing to Holy Island and the port for Ireland.

Welsh Marches Line

The line north from Newport, pictured, to Shrewsbury straddles the border all the way to Shropshire's county town. It passes through the modestly dramatic frontier country that Bruce Chatwin celebrated in On the Black Hill.

Cambrian Coast Line

Birmingham International to Pwllheli: yes, there is a direct train from the home of the NEC and Birmingham airport to the end of the line at a small seaside resort. It gets properly into its stride after Shrewsbury, winding through the hills of mid Wales before opening up to the coast. You sometimes get a taster of the scenery ahead, as the shore curves around to reveal a glimpse of Portmeirion here, the Lynn Peninsula there. You can even change trains from the resort of Aberystwyth to a Pwllheli-bound train at a station with no road connection: Dovey Junction.