Train journeys are expected to start later on Tuesday following the latest round of strikes by rail workers.
Members of the RMT union who work for Network Rail ended their latest walkout over pay and conditions at 06:00 GMT.
The later start means many trains will not begin running until at least 09:00 and as late as midday in some places.
Network Rail said 70% of services would operate, but it has urged passengers to check their travel times with train companies.
“Staff is returning to work, so it is [a] much later start-up for passenger services,” a spokesman for Network Rail said.
“Passengers should check before they travel, so they don’t just turn up at a station for no trains to be there.”
ScotRail said it aims to restore some services from about 07:15 but warned there would be some disruption throughout the day.
“Routes across the country will see services start-up later than normal due to signal boxes opening at different times following industrial action,” ScotRail said.
Thousands of RMT members across Network Rail, which operates and maintains the rail system, and 14 train companies have been engaged in strike action over pay and conditions.
It coincides with wider industrial unrest across a number of sectors, including nurses and ambulance drivers as well as Border Force workers at six of the UK’s biggest airports.
Employees are walking out overpay when the rate of price rises, or inflation, is running at a 40-year high.
Rail strikes will restart between 3 and 4 January as well as from 6 to 7 January.
In the meantime, there is an overtime ban by RMT members at 14 train companies scheduled to run until 2 January.
Network Rail hopes service levels will increase to 90% in the days ahead.
However, it expects this will drop to 20% of standard services once the next set of strikes begins in early January.
Mick Lynch, general secretary of the RMT, said: “The union remains available for talks to resolve this dispute.
“But until the government gives the rail industry a mandate to come to a negotiated settlement on job security, pay and condition of work, our industrial campaign will continue into the new year, if necessary.”
A spokesman for Network Rail said: “The only way we’re going to resolve any industrial dispute is by negotiation and talking, so of course, we’ll look to sit round the table.
“It is just unclear where we can go, as all the cards we have to deal with are already on the table.”