'Witch's hat' stalls four Tube trains in north London
Tube passengers had to walk through tunnels in the early hours after a Halloween hat was thrown on to the track, London Underground (LU) said.
A passenger apparently threw a witch's hat containing metal on to the line at Chalk Farm, in north London, causing a power failure which stopped four trains.
Hundreds of passengers were stuck on the trains for more than an hour before they were walked along tunnels to the nearest stations.
LU has apologised to passengers.
"We apologise to passengers who were inconvenienced as a result of this incident, and would urge customers to exercise care and not to discard items on the railway," a LU spokesman said.
The four trains - three northbound between Camden and Belsize Park, and one northbound between Camden and Kentish Town - were stalled at 0040 BST.
"The metal part of a hat thrown on to the rails at Chalk Farm caused power interruption," LU said.
There were about 70 to 90 passengers on each train trapped for between an hour and an hour-and-a-half.
The London Ambulance Service were called out but no passengers required medical attention.
A number of Tube trains have broken down in recent weeks leading to passengers being walked though tunnels.