Nigel Holness, London Underground's operations director, said:
"On Monday afternoon a Bakerloo Line train terminating at Queen's Park station mistakenly took one passenger, who had not alighted despite several announcements that the train was out of service, into the train depot."The passenger left the train and attempted to make his way back to the station, and was escorted back by the train driver."Every day there are thousands of train movements to and from depots and this type of incident is very rare given the procedures that are followed and safety systems we have in place.
"However, this can happen should a person not hear the three announcements that are always made before a train is taken out of service. In these circumstances anyone that is found will be escorted back to a station or depot."
RMT general secretary Bob Crow said the child, who strayed onto tracks on the Bakerloo Line, could have been electrocuted.
"With London Mayor Boris Johnson mouthing soundbites about driver-less trains and de-staffing the Tube at the Tory conference, yet again only the quick intervention of a driver prevented a 12-year-old boy, who had managed to shimmy up and over the inner car barriers and out of the train, from getting hit by a moving a train or electrocuted on the rails.
"Management have rightly called an investigation into this shocking incident but this does not go far enough for RMT safety reps. We want a meeting to review the whole de-trainment process and a return to a safe way of working."
A tube driver has averted a "potential tragedy" by rescuing a 12-year-old boy who had strayed onto tube tracks on the Bakerloo Line.
The Rail, Maritime and Transport Union says the child narrowly missed the 430-volt live rail, after jumping onto the tracks from a train that had pulled into sidings following its final passenger stop.
It's the third case in as many weeks of a child getting on to tracks on the Underground. Two weeks ago, an eight-year-old was led to safety by a cleaning contractor after being spotted on tracks near Stanmore. Last week a 12-year-old girl was hit by a tube train in Brixton.