Hundreds of thousands of London commuters are facing disruptions during a five-day strike on one of Britain's most problem-plagued rail lines.
Southern Rail workers walked off the job Monday over plans to remove conductors from trains.
Mick Cash, the general-secretary of the Rail, Maritime and Transport Union, said the plan would mean "jobs and safety are compromised on these dangerously overcrowded trains."
But train operator Govia Thameslink said the strike was "completely unjustified." It said it hoped to run 60 percent of scheduled services during the walkout.
Southern, which runs trains between London and its densely populated southern commuter belt, has seen months of one-day strikes and cancelations that it blamed on staff shortages.
London Mayor Sadiq Khan has said Transport for London should take over running the service.