A section of the Central Line was suspended just before the evening rush hour after a woman was dragged under a train when her coat became trapped in the doors as it pulled away.
Twitter user Shabana Khan tweeted that passengers in the carriage heard a "thud" as the train pulled away.
"She's alive and stuck under the train yelling help," she added.
A spokesman for the British Transport Police (BTP) said: "On arrival at the station BTP Officers together with support of the ambulance service, London Fire Brigade and HEMS Air Ambulance located a female who had sustained injuries. She has been conveyed to hospital where her injuries will be assessed.
"The incident is being investigated but is not being treated suspicious, BTP Police officers are conducting enquiries to identify and advise her next of kin."
The westbound Central Line was suspended between Marble Arch and White City, but was later reinstated with severe delays.
Meanwhile, a signal failure at Finsbury Park left the Piccadilly Line with severe delays between Acton Town and Heathrow/Uxbridge, and eastbound between Acton Town and Cockfosters, leaving the rest of the line with minor delays.
TfL said tickets will be accepted on local bus services, while Central Line passengers can use their tickets on Great Western Railway, Greater Anglia, TfL Rail and Chiltern Railways.
The delays came just days after TfL introduced a new timetable on the Northern Line, doubling the length of its highest frequency "evening peak" services in Central London to tackle overcrowding.