Rail passengers are set to be hit by a series of strikes in the run-up to Christmas.
Northern staff began strike action at 07:00 GMT on Saturday in a long-running dispute over guards on trains.
Meanwhile, strike action planned by workers at bus firm Arriva in the north-east of England has been called off.
The decision to call off the bus strike action was announced late on Friday with Arriva making revised offers.
Northern members of the Rail, Maritime and Transport (RMT) union have agreed to strike on every Saturday in December.
Northern said there would be "very few" trains after 17:00 GMT with all services ending at 19:00 GMT.
The RMT said Arriva Rail North, which operates Northern services, had asked the government to reimburse £24m revenue lost through the industrial action.
A Department for Transport spokesperson said any figure was "complete speculation".
They said nothing had been decided, the application was still being considered and any details would be confidential.
"The RMT is ending the year as it started it - causing maximum disruption to passengers with more needless strike action, harming the economy of the north of England and damaging our railways," the spokesperson added.
"This action isn't about jobs, safety or looking after customers, and the RMT knows this."
About 650 staff at Arriva Durham County in the North East had been prepared to walk out on Sunday as part of the Unite's union's call for a wage rise of £1 per hour.