An immediate audit of storage facilities on the Underground has been ordered amid suggestions an obstruction on the line caused a rush-hour train derailment.
Eleven tube passengers were taken to hospital and hundreds were trapped in a tunnel for two hours following Thursday's incident on the westbound Central line between Mile End and Bethnal Green stations in east London.
The Rail Accident Investigation Bureau has begun an investigation into the possibility that a "bale of material" caused an obstruction on the track.
Howard Collins, deputy chief operating officer for London Underground, said: "Initial reports from the driver are he saw something white flapping in the tunnel, before he hit it. There was a loud noise and then he hit the brakes."
Metronet, which is responsible for maintaining the tracks, has announced it is carrying out an examination of all its storage facilities along the tracks and will have completed the checks before the start of service on Friday.
A spokesman said: "Whilst it is important not to pre-judge the outcome of the investigation, our initial reports suggest that a bale of material became dislodged from its licensed storage position in a tunnel cross-passage. As a precaution, Metronet has ordered an immediate audit of all similar storage sites used by its staff and contractors."
All 700 people from the train, and another train stuck behind it, were led to Mile End station. Thirty-seven passengers were injured, 11 of whom were taken to hospital suffering from minor injuries, including panic attacks, breathing difficulties and an ankle injury.
Passenger Jacqui McElroy, 34, said: "It started to rock unbelievably, really violently, the carriage filled with smoke and we could see sparks and there was a smell of electrical burning as the driver was braking."
Both Mile End and Bethnal Green underground stations remain closed, and there is no service on the Central line between Liverpool Street and Leytonstone.
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Thursday's incident site is only a few stops from Chancery Lane station on the Central line where 32 passengers were injured in a derailment in January 2003. The immediate cause of that derailment was a gearbox failure which led to a traction motor detaching.
