After three days of travel misery on Channel rail services, empty test trains gingerly started running as Eurostar engineers worked to solve the puzzle of multiple breakdowns between London and Paris.
However, as thousands of travellers counted the cost of ruined trips, Eurostar confirmed tonight that it will not run any passenger services tomorrow.
Passengers were still being turned away at St Pancras, but the company arranged for 500 elderly travellers and families with children to use ferries from Dover instead.
Meanwhile, a marathon jam on the M20 and other roads to the Channel ports eased, with the first lorries for more than 12 hours embarking for the continent.
Snow and ice continued to be a problem elsewhere, and the A19, through Teesside, was blocked for most of today by a jackknifed lorry. Snowstorms drifted from southern Scotland and the north-west across to the North Sea. More snow is expected in the north and down the east coast to Kent tomorrow. Police in northern counties, East Anglia and Kent appealed to people not to travel unless their journeys were essential.
Manchester airport had to divert a flight from Qatar to Birmingham, as staff twice cleared and de-iced the only runway in use over the Christmas period.
Eurostar's chief executive, Richard Brown, visited queues at St Pancras, telling frustrated customers he was "very, very sorry". He admitted the situation had taken too long to resolve. Passengers trapped underground in trains with broken lavatories, temperatures above 25C (77F) and only emergency lighting criticised train staff's response as "terrible".
Engineers are still uncertain why Eurostar's trains failed, after running unaffected in most previous cold snaps. The company blamed the sudden change between freezing temperatures above ground and the heat of the tunnel. A spokesman said screens to stop snow getting into the electrics had failed.
Eurostar commercial director Nick Mercer apologised to passengers but said engineers believed they had found the cause and would work on the trains overnight to try to solve the problem.
If testing of the modified trains proves successful on Monday, an announcement will be made about when services can resume, he added.
"We have run three test trains today," he told BBC News. "They ran successfully but the engineers on board have recommended strongly that, in light of further snowfalls that are happening tonight, we make some modifications to the trains on snow shields to stop snow being ingested into the power car.
"That work is already under way and will commence overnight.
"We will be testing the trains tomorrow with the new modifications in deteriorating snow conditions."
The inquiry will also look at incidents in the winters of 2003, 2002 and 1995 when a similar sudden temperature change was blamed for electrical faults. Eurostar reported in 2003 that condensation formed, interrupting supplies from overhead power lines, when trains plunged into the hot and humid tunnel from freezing stretches overground. Unlike this weekend's incidents, some trains managed to crawl through the tunnel from London, but lost power on reaching northern France. The current failures were in London-bound trains, which spend longer overground in France before the tunnel than trains going south. Slower freight trains were not affected.
"It's all a bit of a mystery and the company, and indeed a lot of people, appear baffled by it," said Nigel Harris, managing editor of Rail magazine. "The fact that the problem affected London-bound trains rather than ones leaving St Pancras may have been due to those heading away from London having less time to get cold.
"They pass through miles of tunnel through London and then go under the Thames and they just don't have time to get exposed to the very cold temperatures before going into the tunnel. But what is really puzzling is that it is happening now."
French TGV high-speed trainshave not suffered from cold weather problems, although they do not face the abrupt temperature contrast.
On the roads, the AA attended some 7,000 breakdowns by mid-morning today, the same as during the whole of a normal Sunday.
The junior pensions minister, Helen Goodman, reminded neighbours to keep an eye on elderly and vulnerable people. She said: "We've spent £30m this week on winter fuel payments of £250 or £400 for the over-80s, and increased cold weather payments of £25. Everyone entitled will get them and should not worry about turning up their heating."