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."
More top news
Flames could seen leaping from a manhole as parts of Holborn were shrouded in thick black smoke.
A 31 year old man, who was part of a cyber crime gang, has been ordered to repay £1 million or face a further four years in jail
Love Pac-Man? You might be be about to love it a little bit more.