Commuters are facing disruption on the London Underground (LU) after an entire line was closed amid safety concerns.
The closure could affect more than 660,000 passengers
The Northern Line, which carries 660,400 people every weekday, has been suspended and LU bosses have given no indication when it will re-open.
Union bosses called for trains to be withdrawn on Wednesday, after one ran through a red light at Mill Hill East, and its emergency brakes failed.
It was the fifth incident on the line, prompting many drivers to stop work.
Passengers trying to get home on Wednesday struggled to get onto overcrowded buses, after the gates closed at Northern Line stations.
Clare Runacres watched eight packed buses pass before one heading in her direction stopped. She said passengers were "very angry".
"When I finally got on one, it was a scrum at every stop, the buses were all full beyond capacity. It was a very difficult journey home," she told BBC London.
There have been delays on the Northern Line all week because of safety concerns.
Both rail unions, Aslef and the RMT, said they would support any members who refuse to work on safety grounds. Up to 60 have so far refused to work, a BBC correspondent said.
Aslef has since said it will ballot members over industrial action on Thursday if drivers are sent home without pay.
RMT secretary Bob Crow said: "Our members have the legal right to refuse to undertake duties that would put themselves or the travelling public in danger."
Steve Grant, Aslef representative, said: "We are not satisfied that the modifications have got to the root of the problem."
Both unions were due to meet LU representatives on Thursday.
A spokesman for LU said: "We are working with our trade unions and staff to reassure them that it is safe to continue to run Northern Line trains."
He said the company was working with the private sector firms which maintain the Tube to resolve the issue.
LU has used emergency powers to oversee remedial work by Tube Lines and its subcontractor Alstom.