The 49-km blue line, the longest and the busiest in the Delhi Metro network, links Dwarka in west Delhi with Noida and Vaishali on the outskirts of the capital.
According to a Delhi Metro official, the trouble began at around 8.30 a.m. "There was a voltage fluctuation problem due to which the trains were running at slower speed in blue line. The problem was sorted out in 15 minutes.
"Due to the delay there was bunching of trains so trains coming from Noida and Vaishali to Dwarka were slowed down."
He said regular announcements were made at stations about the technical problem and the running of trains.
Trains were bunched, with each coming after every 11 minutes or so.
"The crowd was too much," said an exhausted Rumu Banerjee, who boarded a packed train from Laxmi Nagar in east Delhi. "The compartment I was in was crammed as everyone wanted to get into the train."
Added Rohan Das, another commuter: "It took me an hour from Noida to Rajiv Chowk. The journey normally takes just 25 minutes."