Rails expand and contract according to the temperature (11mm per degree C per km). They are continuously welded together and installed under tension and heated to a neutral / median temperature for the location. It was around 0C that night in an area that gets up to 47C (and rails might get hotter under the sun) so there was at least 300mm of contraction per kilometre of rail.

The rail fractured into pieces during the derailment. You can see some of those pieces lying around in the photographs.

As the article notes: the initial break left marks on the wheels of several previous trains. The final gap is big enough that no train could possibly make it past it, so it is pretty clear that the gap got larger as the incident progressed.