The train derailed between Barons Court and Hammersmith stations
The track section containing a piece of rail thought to have caused Friday's derailment on the London Underground was inspected less than 24 hours before the accident.
The damaged rail has now been removed for tests.
Staff from the maintenance company Metronet walked the stretch of track on a routine patrol on Thursday but failed to notice any major problem.
Metronet is one of two companies responsible for Tube repairs under the controversial part-privatisation of the network.
A London Underground (LU) spokesman said: "At this early stage in the investigation, it appears that a broken rail was the cause of the incident.
"The track was walked and a visual inspection of the rail was undertaken.
"Some minor defects were spotted. They were acted upon and were not related to this incident.
"There was a little bit of rust on the rail but it was not like the rail was crumbling. The rail sheared where it broke.
The "integrity" of the rail and what caused it to crack were under investigation, he added.
The spokesman said the section of track was normally inspected every 72 hours, and stressed that the same regime was in place before Metronet took over the contract.
But the Rail Maritime and Transport (RMT) Union said there used to be daily visual inspections which were cut to every other day just before the maintenance contracts were awarded.
Bob Crow, general secretary of the RMT, has called for Metronet's contract to be suspended until London Underground completes its accident investigation.
Mr Crow said: "We warned that fragmentation of London Underground infrastructure would compromise safety.
"They have only had their hands on this contract for a matter of months.
"Surely the mayor, Transport Commissioner or the government have the power to suspend these contracts pending an investigation."
The rear carriage of the Piccadilly Line train came off the tracks in west London on Friday evening.
Piccadilly and District Line services returned to normal on Saturday as London Underground (LU) began an investigation into the incident at 2117 BST on Friday.
The steel rail is believed to have broken beneath the eastbound Piccadilly Line train as it approached Barons Court station in west London at about 15mph.
The train has been removed from the tracks and is also being studied as part of an investigation.
Some wheels on the last carriage slipped off the tracks, bringing the train to a stop with a jolt and witnesses described hearing a "big bang".
Witness Samantha Ceranka said: "Sparks lit up the whole carriage, the lights went out for a bit and there was loads of smoke everywhere.
"It was very scary. People started to panic a bit. All of us wanted to get out of there."
It took an hour to move out the 76 passengers who were heading into central London.
No-one was injured in what LU is describing as a "low speed derailment".
The company has apologised to passengers for any distress and disruption caused.
LU spokesman Andy Barr said it was very different to the Central Line derailment in January, in which a train jumped the tracks and hit a wall at Chancery Lane station - injuring 32 people.
Passengers walked to safety
An independent inquiry team found a traction motor beneath the train fell off and hit the track.
He said: "Chancery Lane was caused by a defect with the rolling stock.
"In this case, the integrity of the rolling stock was superb, the train remained upright despite one car being derailed."
A rail worker at the scene told BBC News the steel rail was rusted three-quarters of the way through.
Mr Barr confirmed a small portion of the section which failed had rust on it but the main problem appeared to have been a weakened rail.
"The rail looks like it might have been partly fractured before, but the bulk of the rail broke when the train was passing over it," he said.
LU said the rail was subject to both ultrasonic tests and visual inspections.