Tube drivers on three lines have agreed to return to work after checks were carried out on the braking system.
Drivers stopped working on Wednesday evening
The Hammersmith and City, Circle and Wimbledon branch of the District Line were suspended on Wednesday night when unions raised safety concerns.
Transport for London (TfL) said there were no underlying safety issues and services would return to normal but warned passengers to expect delays.
The Rail Maritime and Transport union said it was "satisfied" with the tests.
A spokesman for the Rail Maritime and Transport (RMT) union said drivers refused to work when Tube managers failed to respond to safety concerns about the emergency brakes.
The traction brake controller incorporates a safety device which drivers have to keep pressed down to make the trains work.
If released the trains automatically stop if, for example, a driver were to pass out.
The RMT said the matter had been resolved after an emergency meeting with London Underground (LU), on Thursday morning, when they were told the entire fleet concerned had been tested overnight.
"Our safety reps have satisfied themselves that the tests had been carried out correctly, a briefing note on the problem is being issued by LUL to all train operators, and the company has apologised in writing to the driver who first raised the problem.
"Follow-up discussions will now take place... on both operational safety and on more effective communications."
TfL said: "Transport for London has always stated there are no underlying safety issues and we are glad that we have been able to clarify this for the unions.
"Service on the affected lines has just resumed but customers are advised that they will experience delays on these lines this evening while London Underground builds up a full service.
"We expect to have a full service operating on all lines on Friday morning."