Commuters are facing major delays as guards stage a 48-hour strike on the Southern rail network.
RMT union members walked out again at midnight in the continuing dispute over guards' roles on new trains.
Last week, the drivers' union Aslef began an overtime ban as well as holding two strikes.
A wave of strikes by thousands of workers is hitting the rail, post and airline industries in the run-up to Christmas.
During Aslef's 48-hour strike on Tuesday and Wednesday last week and a further walk-out on Friday, no trains ran on Southern's network.
Aslef and the RMT are in dispute with Southern's parent company Govia Thameslink Railway (GTR).
Previous RMT strikes have affected about 40% of services but Southern said the Aslef overtime ban would add to the disruption.
Southern has warned passengers no trains will run on some routes, while others will be replaced by bus services.
It said it has arranged ticket acceptance with other train operators and some bus routes to help during the strike.
Gatwick Express services between London Victoria and Gatwick Airport are also being affected.
Safe or unsafe?
Under the changes already being brought in by Southern, drivers take responsibility for opening and closing the doors and guards become on-board supervisors.
However, the RMT fears job cuts and has raised safety concerns.
Aslef has described the changes as "inherently unsafe", while GTR said both the Office of Rail and Road and the Rail Safety and Standards Board had stated drivers closing doors was a safe mode of operation.
Talks at conciliation service Acas between GTR and Aslef failed to reach an agreement on Thursday.
Negotiations ended with both sides claiming to be open for talks.
The RMT was told it would not be part of the talks.
On Sunday, the RMT's general secretary Mick Cash denied the strikes were being used to "bring the government down".
And this week, East Worthing & Shoreham MP Tim Loughton will put forward proposals for a new Rail Ombudsman and increased financial penalties against train operators in a motion in the Commons.
The Conservative said penalties currently depend on passengers claiming compensation, and he wants to see automatic levies every time a train is late or cancelled.
"It would apply to the whole rail industry but in practice we have seen what has happened with Southern where there is no incentive for them to get their act together because they are not being hit in the pocket," he said.
The RMT is planning further stoppages by conductors, including over the new year.
Train drivers, who are represented mostly by Aslef although the RMT also has driver members, are set to walk out for six days from Monday 9 January.
Planned Southern strike dates
00:01 Saturday 31 December to 23:59 Monday 2 January (RMT conductors' strike)
00:01 Monday 9 January to 23:59: Saturday 14 January (Aslef and RMT drivers' strike)