Southern Railway will be at a standstill again because of another strike by drivers, with a series of further walkouts being planned in bitter disputes over staffing.
Members of Aslef will strike for the third time this week, bringing fresh misery to the company's 300,000 passengers.
Southern is again laying on 200 buses and coaches to take passengers to railway stations served by other operators, but advised people not to travel unless it was absolutely necessary.
Many people drove to work in London and elsewhere across southern England during the strikes on Tuesday and Wednesday, but wintry weather is expected to add to travel problems today.
Aslef members will also strike for three days later in the month in protest at driver-only trains.
The Rail, Maritime and Transport union announced that guards will stage another 24-hour strike on January 23, the day before the next Aslef stoppage.
RMT members have staged dozens of strikes after Southern changed the role of guards, with the union complaining safety was being compromised.
RMT leader Mick Cash said Charles Horton, chief executive of Southern's owner, Govia Thameslink Railway, had promised during a TV debate earlier this week to retain a second safety-critical member of staff on trains but the pledge "melts away" during direct talks.
"That is the main reason why we are forced again to put on further strike action.
"It is down to the company to end this posturing and to get back into the room with us to kick-start the negotiating process which is what the public are clearly crying out for.
"There is a golden opportunity between now and the next phase of action to get serious and genuine talks under way," he said.
A Southern spokesman said Mr Horton invited the RMT to fresh talks and made a formal written offer to Aslef over a week ago, but had heard "absolutely nothing".
He added: "This wholly unjustified industrial action is causing utter misery and hardship to the travelling public and is having a significant impact on people's work and family lives and the regional economy.
"We remain ready to meet the RMT leadership, as we do Aslef, any time, any place, anywhere to find a way to end their disputes.
"These strikes are not about safety; it's purely about the unions trying to turn the clock back, hang on to outmoded working practices, which technology now eradicates, and union power.
"Every train that previously had a conductor before January 1 now has either a conductor or a safety-competent on-board supervisor rostered to work."
GTR is taking fresh legal action against Aslef by going to the Supreme Court to try to stop the strikes, after losing a court case and an appeal last year.
:: Industrial action by London Underground workers will be escalated from February 6 unless a dispute over jobs and ticket office closures is resolved, the RMT warned.
:: British Airways cabin crew are to stage a 72-hour strike from January 19 in a dispute over pay.