A 48-hour walkout over driver only operated trains continues to cause havoc as it entered the second day of no services.
The RMT has been staging separate strikes over months due to changes to the role of the conductors, with more walkouts planned around Christmas.
Southern has said no trains will run today and has told passengers not to travel. Around 300,000 people usually take 2,242 Southern services every weekday. The strike is due to finish at midnight, though there is another 24-hour strike planned for Friday.
A restricted Gatwick Express shuttle service to and from London Victoria is running every 30 minutes, while Thameslink is operating a normal timetable.
It comes as transport secretary Chris Grayling refused to rule out anti-strike efforts to prevent a repeat of the 48-hour action that has crippled the rail network. Speaking on the BBC's Today programme yesterday, Grayling said he will "have a careful look" at the possibility of legislation to prevent strikes on the railways.
"I can't change the legislation between now and tomorrow. There'll be a lot of things we'll have to take a careful look at," he said. Grayling added that there were issues on the line going beyond industrial action. "I'm not happy with the performance of the railway generally."
He said: "I don't have the power to step in and order people back to work. It is, unfortunately, a lawful strike."
Southern rail's parent firm Govia Thameslink Railway failed in its bid to secure an injunction to stop the action. Judges upheld last week's ruling after GTR appealed the decision.