Rail services reduced on Christmas Eve
Train passengers making journeys before Christmas are facing delays and cancellations on several lines.
First Capital Connect, Chiltern and Merseyrail are likely to cut up to 25% of services on Christmas Eve and nine other operators reported disruption.
The RMT transport union said it was "scandalous" firms could run special timetables without financial penalties.
But the Association of Train Operating Companies said 90% of scheduled trains were running and 80% were on time.
Meanwhile, the Met Office has warned motorists of icy roads across the country.
BBC forecaster Carol Kirkwood said temperatures would plunge to -15C on Friday night, with further snow likely to return to the UK on Boxing Day.
Atoc said the reduced rail timetables had ensured scheduled services were able to run on time.
Director of corporate affairs, Edward Welsh, told the BBC: "It's obviously not great that they've reduced the number of services but this is an attempt by train companies to ensure they keep as many trains running as possible."
An Atoc spokeswoman said many of the timetable alterations were "tweaks", and that many affected services were commuter lines that were not expected to be busy.
"We understand passengers' frustration when there is disruption and apologise to anyone caught up in it," she added.
Delays have also been reported to some routes on services including East Coast, East Midlands, First TransPennine Express, National Express East Anglia, Northern Rail ScotRail, South West Trains, and Stansted Express. National Rail Enquiries has more information.
RMT leader Bob Crow said: "It's a scandal that the private train operating companies can simply chop 25% of their services, install a 'special' timetable and avoid financial penalty.
"This racket shows that the private rail franchise system is rigged in favour of the train operators and means that they can provide what services they like, when they like, leaving passengers out in the cold."
Transport Secretary Philip Hammond said the government's ministerial resilience committee met on Thursday to discuss its continued response to the severe weather.
He said: "We will of course be in regular contact with transport operators, local authorities, and the devolved administrations over the coming days... to keep Britain moving and minimise disruption."
Train punctuality fell during the first bout of bad weather this winter, according to Network Rail figures.
They showed that, on average, 81.7% of trains ran on time in the period from 14 November to 11 December 2010, compared with 89.4% in the same period in 2009.
The punctuality of some train companies fell to about 70% in the early winter 2010 period, but Chiltern Railways managed to run 94.4% of trains on time, making it the best performing company.
The poorest-performing company in this year's early winter period was West Coast operator Virgin Trains, with punctuality falling from 83.6% to 70.4%.
UK airports, meanwhile, are returning to normal as a backlog of flights continue to be cleared.
At Heathrow, British Airways is operating all long-haul flights and nearly all short-haul services.
But there were delays and cancellations at Birmingham, Aberdeen and Edinburgh airports because of bad weather in Europe.
People hoping to fly from Edinburgh and Glasgow to the Western Isles were being bussed to Ullapool to get the ferry to the islands because freezing conditions had closed the runways at Stornoway and Benbecula.
Dublin Airport reopened on Friday morning after heavy snowfall delayed 270 flights and left some 40,000 people stranded.
Two thousand travellers were stuck at Charles de Gaulle airport in Paris as freezing conditions forced half the flights to be cancelled, and snow also hit Belgium, Germany and the Netherlands.
On the roads, police and coastguards have been called out to help a number of drivers stuck in cars after a snap blizzard hit the Whitby area of North Yorkshire.
The Highways Agency advised motorists to take extra care due to the weather conditions, with low temperatures forecast overnight and over the weekend.
A spokesman said: "We have had no significant incidents on the network. Traffic levels have been steady throughout the day."