A five-hour Tube strike on the London Underground line serving Heathrow Airport has begun.
RMT union members on the Piccadilly Line began industrial action at midday on Wednesday in a row over working conditions.
The first 48-hour wave of disruption ended on Friday morning, but drivers took action again at 20:30 BST.
Transport for London (TfL) said the three days of disruption to customers was for "no good reason".
But the RMT claims some drivers say they were threatened with the sack for taking time off after people had killed themselves on the tracks while they were working.
The second strike will also affect the Night Tube.
Talks at the conciliation service Acas ended between TfL and RMT without agreement on Tuesday.
In a leaflet explaining reasons for the strike, RMT said: "London Underground are working drivers on the line into the ground.
"We are striking to be treated with dignity and respect at work and not to be threatened with the sack for taking time off following issues such as suicides in the course of our work."
RMT said this and other concerns members meant it now doubts how "decent" and "safe" the service is.
"This strike is not about money," the leaflet added. "All our drivers taking industrial action will lose a day's salary."
The union's general secretary Mick Cash accused Tube bosses of "wrecking the planned Acas talks" by "refusing point blank to make serious progress on the core issues that have reduced industrial relations on the Piccadilly Line to a powder keg".
TfL's director of network operations for London Underground, Nigel Holness said RMT's claims that no progress had been made were "totally untrue".
"They have made no attempt to engage with us to try and resolve this dispute," he said.
"We continue to uphold all our commitments following discussions with the RMT earlier in the year."
Mr Holness added that he was "hugely disappointed" that customers have had to "suffer three days of disruption for no good reason."
The Piccadilly Line serves Heathrow Airport and carries about 500,000 passengers every day.
Drivers on the line were previously due to walk out in July but the action was suspended following last-minute talks.
There will be no Night Tube running on the Piccadilly Line tonight, but all other Night Tube lines will run as normal.
Services will resume to a normal level at about 07:30 on Saturday between Heathrow and Cockfosters, and about 08:30 from Uxbridge to Acton Town.