But some passengers could not wait to reach their romantic destinations. According to Eurostar insiders - who have nicknamed the route the "Chunnel of Love" - Britons are flocking to join the Mile-Low Club.
Yesterday, a couple, both aged 26, said they were determined to celebrate Valentine's Day by making innovative use of the wash-room facilities during the underground section of the Eurostar journey to Paris. They declined to be named.
"Though the train takes about three hours, you only get about 20 minutes in the tunnel, so you can't waste any time," said the young man, who is already a member of the Mile High Club, courtesy of Garuda Airlines.
"Everyone seems to be doing it. When one of my friends and his girlfriend went over at New Year, he said he couldn't find a free toilet anywhere."
His girlfriend, an old hand at sex below sea-level, added that "the rocking of the train helps to disguise any noise or movement." Being a foot passenger was preferable to travelling by car on Le Shuttle, she added, "because you worry that people might be able to see into the car".
The Mile-Low Club's origins may be down to immigration staff, who were reported last year to have formed a 40ft-Under group. Last night, a Eurostar spokeswoman said the company had received no "official verification" that the train's toilets were being misused. But she added: "The toilets are really quite spacious compared with those on airlines."
However, the bad news for mile-low club members is that at its deepest - 11 minutes after entry - the tunnel reaches only 377ft, well short of the 5,480 feet in a mile.
Passengers wanting to make practice runs might try London Underground. Insiders recommend the longer stops, such as Highgate to East Finchley on the Northern Line, and Perivale to Hanger Lane on the Central.
"Or there is always the circular turn-around at Kennington," said one Mile-Low Club expert. "If you just stay on the train after the 'all change' message, when it disappears into the sidings, you can be left in peace for ages."