Some of the capital's busiest rail routes in and out of London Waterloo will be disrupted by another strike on New Year’s Eve.
Members of the Rail, Maritime and Transport union (RMT) on South Western Railway (SWR) walked out at midnight on Sunday.
They will strike for 24 hours in a long-running dispute over guards on trains.
The rail operator said a reduced service would be run across the network affecting the whole of the south of England.
SWR said it would "do everything... to keep customers moving and reduce disruption”. Some bus replacement services have been put in place.
A spokesman said: "The RMT continues to play politics with their ongoing strike action, causing misery at a time when people just want to be with friends and family or get to work.
“If the union really cares about passengers it should call off these strikes and start helping to build a better railway.
"We have guaranteed to roster a guard on every train - we are employing more, not fewer guards. We remain focused on running services and delivering the improvements our customers deserve including investing £1.2 billion over the course of the franchise.
"We will do everything we can to keep customers moving and reduce disruption whenever the RMT strikes."
RMT general secretary Mick Cash said: "Official parliamentary answers have shown that the Government is preparing to bail out South Western Railway for revenues lost as a result of strike action.
"Passengers will be outraged to know that instead of taking steps to resolve this dispute rail ministers are instead prolonging it by using tax payers' money to prop up South Western Railway on strike days."
