Commuters are facing disruption as workers on South Western Railway (SWR) begin a 27-day strike.
It comes after talks between the Rail, Maritime and Transport (RMT) union and SWR over a long-running dispute over guards on trains broke down.
The operator called the action “unnecessary”. It said “more than half” of weekday trains will run and warned of queues at stations.
The union said the strike was “in defence of passenger safety”.
The RMT said SWR “point-blank refused” to show any serious movement at talks held at the conciliation service Acas.
The union has been demanding that guards should oversee the operation of doors and perform other safety functions in dispatching trains.
It says the company’s proposals would leave guards as “glorified porters” without any safety responsibilities.
Assistant general secretary Steve Hedley said members were “absolutely furious” with SWR following the Acas talks.
“Of course our members don’t want to lose a month’s money running up to Christmas but they’re prepared to do that to show that safety and accessibility for disabled people is non-negotiable.”
SWR said it has offered “a guard on every train, and a safety critical role for that guard.”
Managing director of SWR Andy Mellors said the action was “unnecessary” and the issue needed settled before a new fleet of modern suburban trains is introduced next year.
“Our assessment is that by having drivers opening and closing doors, that will actually optimise the performance of the network by getting more trains to Waterloo on time.
“We’ve been very clear that we’re committed to keeping a guard on our trains and those guards will have safety critical competencies. Our proposals will make guards more customer facing and improve safety, security and accessibility.”
The operator released a revised timetable and said it would provide longer trains to increase capacity where possible.
SWR runs services between London Waterloo and Portsmouth, Southampton, Bournemouth and Weymouth as well as Reading, Exeter and Bristol. It also operates suburban commuter lines in south-west London, Surrey, Berkshire, and north-east Hampshire.
Strike days are as follows:
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