Commuters returning to the railways after the Christmas break face cancelled or reduced services on some routes thanks to staff shortages caused by a wave of Omicron cases.
Today is the first day back for many employees as well as schools after the holidays - though office staff are still advised to work from home if they can.
Operators have been running condensed timetables to try to improve reliability in the face of about one in 10 staff being absent across the industry.
But they have still been seeing an average of about one in 20 services cancelled over the last fortnight, according to the Rail Delivery Group (RDG), which represents Britain's train companies.
Southern Railway, which runs routes through Sussex, Surrey, and parts of Kent and Hampshire said last week that it would be running no trains to or from London Victoria - normally one of the UK's busiest stations - until 10 January.
That also affects services to south London stops such as Clapham Junction, Wandsworth Common and Battersea Park.
Elsewhere, Greater Anglia lists 68 cancellations for Tuesday including on services connecting London Liverpool Street with stations such as Southend Victoria, Ipswich and Colchester as well as the Cambridge-Ipswich and Norwich-Great Yarmouth lines.
South Western Railway - which plans to operate a reduced timetable from 17 January - has withdrawn services between Bristol Temple Meads and Salisbury and reduced services from London Waterloo towards stations such as Portsmouth Harbour, Basingstoke and Poole.
ScotRail said it will introduce a temporary week-day timetable from now until 28 January
More than 150 daily services will be cut, including on routes such as Glasgow to Edinburgh via Airdrie/Bathgate; Glasgow Central to Lanark; Edinburgh to North Berwick; and Edinburgh to Tweedbank.
Great Northern, Thameslink and Hull Trains have also unveiled reduced timetables.
CrossCountry has removed around 50 trains from its timetables between Mondays and Saturdays from 27 December until 8 January.
Latest RDG figures show staff absences running at 9.9% across the industry.
The average rate of cancellations over the last fortnight was 4.9%.
A spokesperson for the RDG said: "We are working hard to provide the most reliable service possible and so that passengers can travel with confidence when fewer rail staff can work, a number of operators are introducing amended timetables.
"There are also some short-notice cancellations so our advice to anyone travelling is to check online before they set out."
Anthony Smith, chief executive of Transport Focus, a watchdog for transport users, said: "What the train companies and Network Rail are doing at the moment is very sensible, they're actually reducing the number of trains they're trying to run and as long as that's temporary, that's okay because it means a more reliable timetable.
"Until rules change about isolation I think we're going to be looking at a patchy situation, over the next two to three weeks.
"So I think passengers need to be really vigilant and keep checking ahead, keep making sure your service is running."