Tens of thousands of commuters faced disruption today when London Underground removed Tube trains from service after engineers discovered suspected cracks on brackets.
Transport for London launched an investigation into the problem on 12 District line trains to discover if it was more widespread.
Passengers faced delays, just two days after a strike by Tube workers over job cuts caused travel chaos in the capital.
A TfL spokesman said: "During a routine inspection of our District line train fleet, our engineers discovered that on a dozen trains a bracket seemed to be cracked.
"Safety is our top priority and, as a precaution, those trains have been removed from service whilst a thorough investigation is undertaken.
"All other District line trains are undergoing testing to ensure that the problem is not more widespread.
"We apologise to passengers for the disruption this is causing this morning, but passenger safety is our top priority and this demonstrates that our safety regime works.
"It also demonstrates yet again the need for ongoing investment in our Tube services and infrastructure, which is of such importance to London and the wider UK economy."
Meanwhile, commuters had disrupted journeys on other Underground lines. An obstruction on the track in the Rayners Lane area of west London led to delays on the Piccadilly line, while another line obstruction - at Barbican in the City of London - caused delays for Circle line passengers.
Faulty trains led to delays on the Victoria line and on London Overground services between Euston station in London and Watford in Hertfordshire, while Docklands Light Railway trains were affected by a signal failure at Canary Wharf.