Severe disruption has been experienced on the London Underground after drivers stopped work over concerns about an emergency braking system.
There are no services on the Circle and Hammersmith and City lines, and the District Line is partly suspended.
Transport for London (TfL) said Aslef and the Rail Maritime and Transport (RMT) unions had raised concerns about the traction brake controller.
TfL said they were working towards resuming services as soon as possible.
The unions and TfL have been in talks since Thursday morning in a bid to solve the dispute.
The traction brake controller incorporates a safety device which drivers have to keep pressed down to make the trains work.
AFFECTED LINES
If released the trains automatically stop if, for example, a driver were to pass out.
RMT general secretary Bob Crow said concern was first raised on Tuesday and drivers do not feel confident taking the trains out.
"The same fault has subsequently been found in at least four trains but seven hours after we asked for an emergency meeting we received a message from the director of safety refusing to meet us over what he described as a storm in a teacup," said Mr Crow.
"It was only after drivers refused to move trains in service that this morning's emergency meeting was finally arranged. It is clear we need urgent checks on the entire fleet."
A TfL spokeswoman said: "As always we are addressing the safety concerns of the trade unions.
"However, these trains have been in safe operation for over 25 years, and there is no reason for the service to the public to be disrupted whilst we seek to address the issues raised."
Services have been suspended on the District Line between High Street Kensington and Edgware Road since 1740 BST on Wednesday.
Trains stopped running on the Circle and Hammersmith & City Lines at 1910 BST and 1940 BST respectively.
A replacement bus service is in place between Paddington and Ladbroke Grove.
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