Some of the most consistent complaints recorded in this newspaper's letters page could fit neatly under the banner of "my train hell!" Missives complaining that a train was too crowded, too dirty and (the most common refrain) too late are as ubiquitous as those vigilant readers straining to hear summer's first cuckoo.

Last week one correspondent condemned Iarnrod Eireann for providing what he called a Third World rail service, and that, compared to other comments, was kind.

Reading these apoplectic musings, regular passengers on the Belfast-Dublin Enterprise smile serenely and can't help feeling a bit smug. Here is a train service that on its fourth birthday is almost unanimously well received by the travelling public and gets record numbers to their destinations.

For some it's the comfortable seating, for others the automated public address system, while sticklers for cleanliness appreciate the presence of on-board cleaners who will gladly relieve you of any rubbish accumulated on the journey.

Some just like to sit back in a tobacco-free environment - the entire service is non-smoking - and enjoy the view. Journey time on the Dublin-Belfast route is a whistle-stop two hours. And despite the occasional line-blocking activities of dissident republicans on both sides (you can judge the state of the peace process by the level of disruption on the Enterprise), it is reliable in a way some Iarnrod Eireann services can only dream about.

"In our view, the success of the Enterprise illustrates the benefits of significant investment in the rail network", said Barry Kenny, of Iarnrod Eireann, who jointly run the service with Northern Ireland Railways. The cross-Border service was established with £120 million of funding, most of which came from the EU with a little help from the British and Irish governments.

In 1990 the route was struggling to maintain fewer than 400,000 passengers a year with an unreliable service and ancient carriages. These days one million passengers a year travel on the three Enterprise trains.

Each features comfortable interiors throughout standard class and even plusher seating in the First Plus carriages. Four years later the trains are holding up well, but even so, in November, those behind the Enterprise will begin a programme of overhaul because, as Enterprise manager Ken McKnight explained, "we want to keep the Enterprise to the highest possible standards".

He added: "We are constantly trying to provide a service that meets or exceeds our customers' expectations and we are never complacent about the service, always looking for market growth, and now we have surpassed even our most optimistic projections . . . We are continuously struggling to keep the fleet in pristine condition."

It is a philosophy that Iarnrod Eireann say it wants to emulate. "There is a clear difference in standard", said Barry Kenny, speaking of the contrast between the Enterprise and some of the State rail company's more maligned routes to Sligo and Kerry.

Long-awaited investment is finally taking place, and over the next six years £1 billion will be pumped into an upgrading of the infrastructure, the replacement of stock and an increase in the overall capacity of the rail network.

"We will be looking at having business managers who champion different routes, much stronger staff involvement, and hopefully greater pride in the service . . . In the Enterprise, you have a very strong brand which inspires confidence in staff", he said.

While the Enterprise has set enviable standards over the past four years, it would be a miracle if things always ran smoothly. Sure enough, this week there was an incident which has caused those in charge to rethink their strategy in relation to the new, improved menu featuring the best of local produce on offer in the First Plus section of the train.

First Plus, with its table service, newspapers and complimentary orange juice, has proved popular with business users and people with a bit of spare cash who like the idea of sipping champagne on a train. As with all the routes in the South, Network Catering (Dubel in the North) is in charge of the food, which includes staples such as steak, omelettes and sandwiches.

On Thursday a group of journalists were invited to sample the new menu, which includes versions of two signature dishes from acclaimed chef Robbie Millar, who owns the Michelin-starred restaurant, Shanks, in Co Down.

A few months previously, Millar had taken the cooks on the train to his restaurant, trained them up and showed them how exactly his penne with salmon and his fillet of beef with champ should be done.

What was presented to him on the train was so far removed from what had been stipulated that a gobsmacked Millar, anxious to protect the good name of his award winning restaurant, has taken his dishes off the menu until the situation improves.

Such unfortunate glitches aside, 2.5 million miles and four years later, the Enterprise has pioneered what disgruntled rail users hope will be the future of train journeys across this island.