Restaurateur Mitch Tonks has given the Great Western Pullman dining car a makeover

We've barely creaked out of Paddington before a leather-bound menu is thrust into my hands, and Royal Oak, the stop a few minutes on, sees the delivery of fresh bread rolls and a pat of farmhouse butter as the seats around me fill in response to a Tannoy announcement. By Reading, I'm in full lunch flow, devouring a plateful of salmon artfully cured by Somerset's Brown and Forrest smokery – and wondering if I'm dreaming.

I'm not – I'm in the Great Western Pullman dining car on the 12:06 from Paddington to Penzance. (The name Pullman refers to George Pullman, who introduced lavish dining cars to US trains in 1868.) As I nibble my salmon starter (£10), I reflect that there can be few more civilised pleasures known to mobile man than enjoying good, freshly cooked food against a backdrop of ever-changing scenery. It certainly helps pass a journey in a way that a novel or newspaper can't – and every human needs to eat, so trains have a captive audience.

It seems extraordinary that while everywhere else, from the high street to airlines and airports, is upping their food game, the only alternative to the trolley or overcrowded buffet car on trains – with the exception of certain First Great Western (FGW) ones – is the at-your-seat airline-style plastic tray (and that's a "privilege" reserved for first class only). Surely you take the train precisely because you can walk around rather than having to be imprisoned in your seat. And what could be nicer than "going out" – or at least "along" – to lunch?

Ever since East Coast trains called last orders on its restaurant cars in 2011 after its predecessor, National Express, made massive losses, rail companies here and across Europe have argued that Pullman dining cars don't pay and that customers don't want them. Even French railways have scrapped them except on certain international services. Bravely – some would say foolhardily – First Great Western kept its Pullman going and has now given it a makeover by employing Devon-based restaurateur Mitch Tonks to develop menus using quality ingredients from within 15 miles of the line. "It started after Phil Edgerton, our head of marketing, ate at Mitch's Dartmouth restaurant and sampled some fresh prawns that Mitch's daughter had just caught," Jo Elliott, FGW spokesman, says. "Phil said: 'Why can't we do this on the train?'"

It might not be quite the Paris-Istanbul Orient Express, whose inaugural 1883 restaurant coach, panelled in mahogany and teak, ushered in the golden era of railway dining in Europe. (The first British train to host a dining car was the Great Northern Railway, which started serving hot meals, cooked over an open fire, in 1879). Our glasses are from ordinary glass, not Baccarat crystal, and tablecloths are paper instead of starched damask linen. But our white china plates with their pukka red Pullman logos, the soothing clinking of glasses and the chatty service all create a pleasing feeling of lunchtime ritual. Plus – and this is a big plus – our sit-down lunch is an excuse for us second-class mortals to enjoy the spacious, leathery comforts of first class.

Clare, our smartly uniformed "service leader" – trainspeak for front of house – has just braved a particularly jolty stretch of line to bring my main. "You get used to the movement," she says, watching the glasses on my table slide gently. I faced a tough choice between South-west coast-landed lemon sole with brown shrimps (£25) or Somerset-reared fillet steak with Café de Paris butter (£28) but the steak has won, if only to give me an excuse to enjoy a miniature pot of Tracklements mustard (made due north of where we now are) and a 500ml bottle of Ramon Bilbao Rioja (£20), which is going down nicely.

As we whizz past a white horse etched on the chalky Wiltshire Downs, I notice an interesting sound above the thundering of the engine and the clinking of glasses: the happy hum of human chat. Having been seated at tables of two or four, my fellow lunchers have struck up conversations, some so animated you'd think that those involved had known each other for years. Next to us a man on his way to his daughter's graduation in Exeter is chatting with a retired gentleman from Rock. Behind, a group of four have stumbled across the Pullman lunch as a delicious way of entertaining granny on the five-and-a-half-hour journey to Penzance. It feels more like a supper club than a British train carriage. And not a laptop in sight.

The mechanics of producing our three-course lunches from the tiniest kitchen imaginable, with just minutes to prepare, are a logistical marvel. Cooking in constant motion is not for everyone, but chef Pete Downham used to work in the merchant navy so is unfazed. Today there's a hiccup – the oven isn't working – so Pete is masterfully making do with a grill. Some of the dining-car food is pre-prepared, of course, but to the company's great credit, fish, steak and vegetables are all loaded in Plymouth and cooked from fresh.

The train has reached the lush pastures of Somerset, so Clare takes our dessert orders. There's no time to waste, as many passengers will be getting out at Exeter and all traces of lunch need to be cleared by Plymouth. I plump for the chocolate-orange mousse, with candied orange peel, at £9. Again, it's a difficult choice, as the artisan British cheese selection looks tempting, too. It's followed by tea (disappointingly, Lipton, but at least it's with real milk) and mints, which I spin out as long as possible as we imbibe the most spectacular views of the journey: Dawlish Warren with its sand dunes, bobbing boats and glistening seas. Our train hovers so close to the water it's hard to believe we're not actually in it.

It's time to settle up. My three courses come to £47. I shared my wine with my companion, so my drink costs an extra £10, making a total of £57. Not cheap, but judging from the feedback on TripAdvisor and my fellow lunchers, not outrageous if you take into account the top-class local ingredients plus the convenience, the entertainment and the chance to travel first class for a couple of hours. Many passengers tell me that they never knew about the dining car but have been pleasantly surprised and will now book trains that have them – only three each way daily on the Paddington-Penzance at present, although there are plans to introduce them on the Paddington-Swansea line next month, too. Others say they're already in the know and pick their trains accordingly. "It's a well-kept foodies' secret," one passenger, visiting family in Newton Abbot, tells me. "It's tempting not to tell anyone in case we can't get in. But I really hope it flourishes."

So, can the Pullman pay its way? FGW's Elliott admits that staffing costs make this difficult. "We have to take a holistic view. We believe the dining car adds to customer perception. We hope it'll become a point of difference, a reason to take the train." From what I've seen today, it might well.

The seagulls herald our arrival at Plymouth, my destination. For once I'm wishing my journey hadn't flashed by so quickly, so I could have a few more hours to savour the Pullman's food and company. Perhaps First Great Western could ask their drivers to drive a little more slowly next time.