The locomotive and the next two vehicles travel over the fracture point without coming to harm but the HSE can tell from damage caused to the underside of the coaches that the rail had already started to disintegrate.
When Mr Winsor announced he would consider withdrawing Railtrack's licence in the aftermath of the Paddington rail disaster, Sir Alastair wrote him a stiff letter pointing out the damage he was doing by voicing such threats in public.
This is a problem for railway bridges too, according to Network Rail, and it employs scuba divers among its maintenance crew in order to spot damage to bridges - not to mention abseilers - to deal with some of its seafront lines.
Network Rail said the spoil heap had stopped moving and it had been able to survey the damage, which it said was "a lot worse than we originally thought".