Commuters on three London rail networks are facing travel chaos as staff walk out for 48 hours, causing services to grind to a halt.
Strikes on Southern, South Western Railway and Greater Anglia lines across Wednesday and Thursday form part of the biggest disruption to rail services in decades.
Members of the Rail, Maritime and Transport union (RMT) are locked in a bitter dispute over the role of guards and driver-only trains.
Two other lines, Merseyrail and Arriva Rail North, are also striking for 24 hours from Wednesday over the same issue.
Hundreds of services across the capital will been cancelled, with replacement buses covering the gaps.
Meanwhile, travel misery was compounded when Thameslink suspended services between Wimbledon and Sutton after a person was hit by a train.
The biggest strike disruption is threatened at SWR, which only took over the franchise from South West Trains in August, with more than a third of services set to be hit.
SWR managers have accused the RMT of calling an "unnecessary" strike because the company has no plans to abolish the role of guards.
Meanwhile union Aslef, which is also in dispute with Southern, will give the result of a ballot of train drivers on a proposed deal later.
The proposed agreement also includes a five-year pay deal worth 28.5%.
The executive committee of the train drivers' union had recommended its members accept the deal.
RMT general secretary Mick Cash said: "RMT members stand solid, united and determined again this morning in the latest phase of strike action in a raft of separate disputes which are about putting safety, security and access to transport services before the profiteering of these rip-off private rail companies.
"Political and public support is flooding in as our communities choose to stand by their guards against the financially and politically motivated drive to throw safety-critical staff off our trains.
"It is frankly sickening that Transport Secretary Chris Grayling and his supporters are prepared to sit back and cheer on overseas operators who are robbing British passengers blind while sacrificing basic safety standards in order to subsidise transport services in Paris, Amsterdam and Hong Kong.
"It's time for the Government to lift the dead hand which is preventing rail companies from negotiating deals like the ones we have successfully struck in Wales and Scotland that guarantee a guard on the trains. If it's good enough for Scotland and Wales it's good enough for the rest of Britain."