The train, travelling at more than 100mph, derailed after passing over a section of rail which then fractured.
The agency said late Saturday afternoon that its investigators had found a section of rail on the eastbound track "that was fractured at a rail joint".
In the morning, a section of rail on the Babylon branch of the Long Island Rail Road near Amityville snapped, said a spokesman for the agency, Brian Dolan.
While the machine was at work in Queens, a worker, far left, guided a section of rail onto new ties, while Web Merrithew manned a control panel that guided new ties along a conveyer belt to the bed.
The Health and Safety Commission had appointed Tony Barrell to head the inquiry into the crash in west London, which happened on a section of rail where Heathrow Express trains will run alongside mainline trains when the airport branch opens next year.
Transit officials say they now believe that a section of rail that pierced the bottom of a subway car on Monday was a damaged piece that had been taken out of service and was sitting on the track bed for more than a year, waiting for pickup.
Seven councils have contributed a total of £45m to help fund the restoration of a section of rail line between Cambridge and Oxford.
A section of rail clad with martensitic stainless steel was subject to bend fatigue testing and ran out to 5,000,000 cycles at a stress range of 350 MPa matching the performance of an unclad R260 rail.
A track circuit is made by the two rails of a section of track, insulated at their ends.
It detects a train's location when the wheels and axles cause a short circuit along a section of running rail -- not when a metal shoe on the train bridges a gap in the electrified third rail, completing a circuit.
