The power went out at the traffic control central overseeing the Swedish capital's busy green lines at 4.20pm on Wednesday, forcing workers to close almost all stations on lines 17, 18 and 19.
"When the signal system doesn't work it's a big deal because traffic control can't see where the trains are," Claes Keisu at Stockholm public transport operator SL told the TT newswire.
More than 100,000 people usually use the green line during weekday rush hour.
Commuters at a busy Slussen metro station on Wednesday afternoon. Photo: Tobias Röstlund/TT
Many travellers were left waiting on trains that were not moving. At the Skanstull bridge in southern Stockholm some people spontaneously evacuated the train at 5pm without permission from staff. Video clips and pictures showed people climbing over a barbed wire fence and walking along the tracks.
"It is dangerous to walk on the tracks but it is understandable that people want to get out. But it is nothing we recommend. We want to do it in a controlled manner," commented Keisu.
Trains were running again by around 6pm, with knock-on delays throughout the evening.
However, the green line was back to normal by Thursday morning rush hour.