Tube travellers had to be led along tracks through an underground tunnel for the second day in a row today because of fresh disruption to services.
Around 400 passengers on London Underground's Victoria line were stuck in a tunnel near Seven Sisters after a defective train caused huge delays in the rush hour.
There were also delays on other parts of the system, including the Northern and Circle lines, because of signal failures and track problems.
The Tube delays followed yesterday's power failure on the Jubilee line which led to thousands of passengers being stranded in tunnels before being led to safety along the tracks.
Some passengers caught up in the latest breakdown posted messages on Twitter, including: "People stuck for 2 hours underground on Victoria Line as another new train breaks down. Have to walk the tracks. This is becoming a scandal", and "Made it to the office, eventually - the Victoria line is consistently awful."
The RMT said an ongoing ban on overtime by thousands of workers in a long-running row over job cuts was having an effect on services.
The union, and the Transport Salaried Staffs Association (TSSA), has already held two strikes and is threatening fresh walkouts next month, claiming Transport for London is planning to cut almost 2,000 jobs.
Talks between the two sides will be held today at the conciliation service Acas.
RMT leader Bob Crow said: "Once again this morning the transport system in London has been reduced to chaos through a combination of mismanagement, financial cuts and a withdrawal of goodwill by Tube staff who are furious at the ongoing threat to another 2,000 jobs. Those job losses would plunge travellers even deeper into the black hole of unsafe and unreliable services.
"The Mayor, who is responsible for transport in this city, appears to have gone Awol and his officials at Transport for London have now resorted to barefaced misinformation and distortion when it comes to admitting the scale of the shambles that has developed on their watch.
"As the system lurches from crisis to crisis we have a golden opportunity at the Acas talks today to reach a settlement that protects long-term safety and security on the Tube and which maps a way forward out of this mess."
A Transport for London spokesman said: "We apologise for the disrupted journeys that some passengers will have faced this morning, caused by a signal failure on the Circle line, a track problem on the Northern line and a defective train on the Victoria line.
"We apologise to customers for any inconvenience they have experienced during this morning's problems, which also demonstrate the need for ongoing investment in our Tube services and infrastructure, which is of such importance to London and the wider UK economy.
"The RMT leadership's overtime ban was only a factor in delays on the Metropolitan line, where the RMT leadership is continuing to try to hamper routine maintenance. We have given the unions cast-iron assurances that our staffing changes are being delivered with no impact on safety, no compulsory redundancies and no pay reductions, that every station with a ticket office will retain one and that all stations will remain staffed. We will be happy to repeat these assurances to the unions' leaderships when we meet at Acas."
The RMT later announced that Tube maintenance staff will launch industrial action short of a strike from next Tuesday, October 26, after they voted in favour of the move in a row over spending cuts.
Mr Crow said: "We have warned repeatedly that LU's cuts plans are playing fast and loose with safety and will turn the Tube into a death trap. It is a scandal that the London Mayor, Boris Johnson, and his transport officials have chosen to ignore those warnings.
"The anger of our members who carry out the safety-critical function of maintaining the Tube fleet, at the cuts being imposed from above, is reflected in this massive vote for action.
"The closure of District line services, and the shortage of rolling stock on other lines, has been a damning indictment of London Underground's cuts plans which has rammed home the consequences of ripping up the rule book on fleet maintenance.
"RMT members have been forced into a position where they have no choice but to take action on behalf of Londoners who depend daily on a safe transport system."
A passenger on a Victoria Line service said he was stranded underground for two hours after his train suddenly ground to a halt in a tunnel between Seven Sisters and Finsbury Park in north London.
"After about 20 minutes the driver walked through the carriage and explained that the engine had stalled. Some engineers arrived and cut off the power, then a couple of paramedics came in and handed out bottles of water.
"There were a couple of pregnant women and an elderly person and someone passed out. Everyone was taken out through the front door and we walked about 100 yards along the track to the platform.
"This was the fourth time in the last three weeks my journey has been disrupted. The Victoria Line has become an utter shambles."