A South Western Railway train partially derailed in south-west London leaving one person with minor injuries and causing significant disruption between Wimbledon and London Waterloo.
Emergency services attended the "low-speed partial derailment" in the Wimbledon area near East Putney, National Rail said.
British Transport Police (BTP) warned of "significant disruption", tweeting: "Officers are attending a low-speed partial derailment of a train in the Wimbledon area. One person sustained minor injuries."
London Fire Brigade said it was called to the derailment at 5.54am, and that crews were helping around 300 passengers from the train after correcting the position of the rear carriage which had come off the tracks.
National Rail expects disruption to last until the end of the day.
The derailment also halted District line trains between Parsons Green and Wimbledon.
Meanwhile, further delays to the London Underground network were caused by a power failure which wiped out the entire Bakerloo line.
Severe delays are affecting the line, after trains were temporarily halted between Queen's Park and Piccadilly Circus.
A signal failure at Barking caused delays to the Circle and Hammersmith and City lines, while a signal failure at Wembley Park caused minor delays on the Metropolitan line.
A local power failure on the Northern Line meant trains were not stopping at Kentish Town, Tufnell Park and Goodge Street, while Angel station was exit only.
Kentish Town has since reopened.
London Overground was part suspended, with no service between Clapham Junction and Wandsworth Road due to a faulty track.
There were also severe delays between Watford Junction and Euston due to a faulty train at Queens Park.
The Rail, Maritime and Transport union said the derailing incident highlighted the importance of having guards on trains.
RMT members on SWR and four other train operators are striking this week over the role of guards and driver-only trains.
RMT general secretary Mick Cash said; "RMT awaits the facts behind the derailment this morning and our local reps are monitoring the situation on the ground.
"Yet again this incident shows the importance of having guards on our trains to manage emergency situations and exposes South Western Railway's dangerous gamble of axing the guard from their services.
"That is the very issue that our members will be fighting over in their action this week."