Eurostar's journey has been a long one - not just in terms of the 495km from London to Paris, but also in terms of the decades between the idea of linking both European capitals and its eventual implementation, followed by a 17-year slog towards profitability.

Part of the complexity comes from the distinction between the Eurostar train service and the tunnel through which it runs.

The high-speed train service connecting London to Paris and Brussels is owned and operated separately from the Channel Tunnel, though the latter had to be built for the trains.

The idea of linking Britain and France across the English Channel had been around since the 1800s, but picked up steam only after World War II.

Britain had worried about the implications of such a tunnel for its defence. But as military air power grew more important, Britain eventually accepted the idea in 1955.

The British and French governments announced their commitment to the tunnel in 1964. After studies, construction was launched in 1973 - then cancelled in 1975 when the oil crisis hit.

When then British Prime Minister Margaret Thatcher got the ball rolling again in the 1980s, she determined that it would have to be privately funded. Then French President Francois Mitterrand agreed.

Construction of the tunnel, whose average depth is 50m below the seabed, began again in 1988. The tunnel opened in 1994, a year behind schedule and at a cost of £4.65 billion - equivalent to £12 billion (S$21 billion) today - 80 per cent more than projected.

The benefits have proven harder to calculate compared with the costs. One issue is that it is hard to isolate the Eurostar's impact from that of the Channel Tunnel itself - which has a separate rail service for vehicles - and of Britain's integration with Europe more broadly.

In 2014, 21 million passengers were transported between Britain and France using the tunnel - up from 7.3 million in 1995.

Of those figures, 10.4 million went on the Eurostar in 2014, compared with three million in 1995.

Eurostar's journey remained complicated after train services began. For instance, the British stretch initially lagged behind - literally.

In France, Eurostar trains had a top speed of 300kmh on high- speed rail tracks. But upon entering Britain, they had to run on existing lines at speeds no higher than 160kmh. This changed when the Channel Tunnel Rail Link was completed in 2007, allowing higher speeds in Britain.

Commercially, Eurostar's history is also a tangled one. It was first run jointly by the national railway companies of France and Belgium, as well as a subsidiary of a British government-owned firm.

In 2010, it came entirely under a single Eurostar corporation that was jointly owned by the railway companies of the three countries.

It was only in 2011 that Eurostar reported its first operating profits.

Then, last year, the British government sold its 40 per cent stake to a private consortium.

Britain's National Audit Office thought the timing of the move "was primarily driven by the desire to sell prior to the 2015 election".

In contrast to Eurostar's bumpy progress, its passengers enjoy a much smoother ride.

"The Eurostar is much more convenient than air travel," said Londoner Faisal Sayood, 27, an actuarial consultant who zips over to Paris or Brussels once or twice a month for work. "The stations on both ends are in the city centres and so don't require the extra hour or so (to get to the centres) from the airports."

Quick security checks, short waiting times and fast border-control clearance give the Eurostar a further edge over air travel, he said.

Most travellers share his preference. By 2004, 10 years after operations began, Eurostar had 59 per cent of the market share for all trips between London and Brussels - in either direction - and 66 per cent for trips between London and Paris.

By 2013, eight out of 10 passengers travelling between London and Paris or Brussels were taking the Eurostar.

Janice Heng