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
As disruption continues throughout the evening here's what you need to know
Fallen trees, damaged power lines and emergency speed limits were today causing chaos for rail passengers.
They will be supported by artists including Zara Larsson, G-Eazy, Wiley, Sean Paul, WizKid and an exclusive set from Bryson Tiller.