Tube drivers on the Victoria line are staging a 24-hour strike in a row over door safety equipment. About 180 drivers on the Victoria Line stopped working from 9pm on Tuesday.
The RMT union claims trains need safety features to prevent doors from opening on the wrong side.
The industrial action is being taken by workers from the Seven Sisters depot in north London. The workers said the correct door-side enabling equipment was installed on all other lines on the London Underground.
General secretary Bob Crow said he was also determined to stop "a culture of bullying and harassment" of RMT members on the Victoria Line.
A Transport for London spokesman described the strike as "completely unnecessary".
"There are procedures in place to deal with these kinds of issues and we ask RMT to return to the negotiating table," he added.