Sarah and Alex Bell are holding hands over a train table.
 
"I liked trains, but I was far more interested in Alex," remembers Sarah fondly, as her husband of two years looks suitably bashful.
 
"It was only when we were overlooking the track, with a Garratt engine coming towards us, that the power and glory of steam suddenly converted me. We married a year and a day later."
 
Blimey.
 
What is it about steam trains?
 
What on earth is a Garratt engine?
 
And why do I think I'm about to find out?
 
Sarah and Alex are sitting in the first carriage of a train on the newest railway in Wales, where the only thing that stops them gazing at each other is the sight of something really sexy, a newly restored engine, say, retrieved from South Africa, where it has done a million miles or so lugging timber across the veld, or a particularly shiny, well-laid set of rails.
 
Oh, and a front window full of rural Wales, passing in a glorious blur of bluebells, spring woods, and sheep velcroed to the green skirts of Snowdon.
 
Making tracks for the lakes at the rear of the train, in first class, where there are free-standing armchairs and the observation car has a single pane of glass carefully curved to avoid unwanted reflections, are the Gold sponsors, people who have given £6,000 or more to make this day happen.
 
In between are 150 Silvers (£200 or more), and volunteers who have given not only money, but days, weeks, sometimes years of their lives, grabbing weekends and holidays, often travelling from other parts of Britain or abroad, to shovel coal, lay track, and lug sleepers.
 
They are all bonkers, obviously.
 
I don't think I've ever been on a happier train ride.
 
The new Welsh Highland Railway, which runs down the western edge of Snowdonia National Park in north-west Wales, has been creeping towards this moment for the past 12 years.
 
It went south from Caernarvon to Dinas in 1997, Waunfawr in 2000, and Rhyd Ddu in 2003, minutely observed by the railway press, the Government, local residents and landowners, the national park, and Visit Wales.
 
Now it is open to Hafod-y-Lyn and later this year it should complete the missing link into Porthmadog.
 
Not only will train buffs and tourists be able to transfer to its sister route, the 13.5 mile Ffestiniog Railway, from Porthmadog up into slate country at Blaenau Ffestiniog, it will also open up Snowdonia to cyclists (the trains have bike wagons), walkers, and the carless.
 
As we pull out of Waunfawr and start climbing, there are exclamations of pleasure.
 
Not just at the scenery, as the great mountain rears up above us with its broad summit lost in cloud, and little grey farmhouses balanced on its lurching fields, but because nobody can quite believe it's happening.
 
The leaders of the track gangs, groups of volunteers who laid the 25 miles of steel rails and sleepers, josh each other over who holds the top-speed record of half a mile in two and a half days.
 
"There's Roland's Runner!" the cry goes up as we pass a contraption invented by a volunteer to move rails.
 
Everyone winces as we scrape against granite rubble dumped too close to the rails, they're worried about Gary, who painted the livery and crests and tricked out the interiors in cream and coral.
 
The Welsh Highland Railway has cost £28 million so far and needs another £300,000 to reach Porthmadog.
 
The five engines, double-ended, designed by a bloke called Garratt I now discover, were shipped back from Africa and Tasmania and restored.
 
All the carriages, bar two originals, have been made from scratch by full-timers and volunteers at the engineering works, Boston Lodge.
 
It's smart, comfortable, and deliberately designed to be a different experience to the Ffestiniog, with its Victorian heritage carriages.
 
"These were always private railways, they were never nationalized. In the Twenties, Porthmadog was like Spaghetti Junction. It was littered with railways, all narrow gauge, all owned by Ffestiniog Light Railways, all bringing slate and minerals into the port," explains Stuart McNair, one of the supervising engineers.
 
The Ffestiniog itself opened in 1836.
 
Slate had previously left the mountain on donkeys, but the new narrow rails could cope with the terrain.
 
Still, carriages were horse-drawn until it became the first narrow gauge railway in the world to use steam, with spectacular results.
 
"There was a massive slate boom in the 1860s and 1870s. The North Wales slate barons became some of the richest people in the country, that's when they built castles like Penrhyn," says Stuart.
 
The poor old Welsh Highland Railway, completed in the Twenties, missed the boom and limped along for 17 years before closing," says Stuart. Fortunately for the post-war preservationists, neither railway disappeared.
 
The Welsh Highland, too costly to remove, gradually gave way to walking and cycling paths and people's gardens.
 
The Ffestiniog, established by an Act of Parliament, could not be demolished without legislation, so it was mothballed.
 
According to Paul Lewin, the general manager of both railways, "They flung open the doors to Boston Lodge and all the engines were still sitting there, untouched."
 
Paul is sitting one carriage up from first class, chewing the fat with engineers, volunteers, and board members.
 
His father built miniature steam railways for amusement parks, "I've got pictures of me aged five, going around and around in circles. You ran them in, like cars."
 
All day I hear similar stories, people whose fathers worked on trains, people whose families holidayed in north Wales, and went on the Ffestiniog for a treat, and people who started at the annual August Kids' Week.
 
About half the 7,600 members and half the management team are women, one female Kids' Week alumnus now drives trains for London Midland.
 
Outside, we have reached the high point of the railway at Pitt's Head, 650 feet above sea level.
 
It's steep, a 1-in-40 gradient (2.5 percent), where a mainline track would rarely be more than 1 in 100 (1 percent), with sinuous "S" bends.
 
That means it can't be used in winter, even now, in spring, it's tough on the engine.
 
But we're over the hill and running for the Aberglaslyn Pass, the first time any passengers have travelled this far, and all attention turns to the landscape.
 
We follow the pretty Glaslyn River as it makes its way down to the Porthmadog flood plain through mixed woodlands smudged with bluebells.
 
Hills and escarpments erupt to the south.
 
Meadows nod with ladies' smocks and buttercups.
 
We pass the village of Rhyd Ddu, one of several places where hikers can leap out and make for the summit of Snowdon and Beddgelert Forest Campsite, one of the biggest in Wales, and an important market for the train.
 
Gelert was the hound of Prince Llewellyn and Beddgelert is as famous as the site of his grave, a corking story made up by the landlord of the Royal Goat Hotel to drum up customers.
 
As we pass an osprey's nest belonging to Wales's first breeding pair, all paths nearby close during the breeding season, so the train is a useful vantage point, the day is coming to an end.
 
Or it is for the triumphant passengers, who have just seen a dream come true.
 
I, on the other hand, am going to make a mad dash across country to catch the last train of the day on the Ffestiniog Railway.
 
The steam, the soot, the Garratt engine... Really, I could get into this.
 
The Sister Railways
 
Only first-class tickets can be booked in advance on the numbers below.
 
Welsh Highland Railway (01286 677018, Easter to end of October, single third-class tickets £6.60-£17.50, depending on stops, returns £9.90-£25.
 
First-class upgrades from £5 one way.
 
Dogs and bikes £2.50 one way.
 
Ffestiniog Railway (01766 516000, open all year), single third-class tickets £7.35 to the halfway point, £12 all the way.
 
Returns from £14.50.
 
When the Porthmadog link is complete, a ticket for both routes will cost £43.
 
Kids' Week, when children can apply to work unaccompanied by adults, doing jobs such as carriage cleaning, building, and painting, and staying in a local hostel, takes place in the first week of every August, but is already full for this year.
 
The Light Railway Fantastic
 
Welsh Highland Heritage Railway at Porthmadog, a separate entity, does return trips from Porthmadog to Pen-y-Mount junction (£15 family tickets last all day, including the miniature railway and newly enlarged museum).
 
Sophie Campbell.