This is the BBC's static news item on his accident, Its content is mostly as in the rolling news item, and it is  still dated yesterday; I would point to this paragraph:

There is also this picture, credited to Reuters, which strongly suggests a major failure of the launch girder system itself. The perspective is confusing - The Times had a better view, I think from Getty, which I can't find anywhere on-line.
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You may remember how such a system works, from the one used on the Colne Valley Viaduct (aka Dominique). The girder must rest on two supports at all times, and it moves forwards along the viaduct either  by a support crawling along the now-built viaduct, or using a third support.

What I see in the picture suggests something along these lines: the forward support has collapsed, the girder has fallen onto a pier ahead of where segments are being added, and it has buckled. The moving crane that carries the segments was operating at the time, and could not sustain the shock as the girder landed; part of that crane, and perhaps the segment, fell off. That forward support can't be seen, so most of it may also have fallen onto the train.

Note that this is not unprecedented - another launch girder operating in Thailand, this one building a road viaduct (and painted blue) collapsed in January 2024. 

 
Latest count 28.

A second crane from the same site fell the following day in the other direction on to a road, and killed two further people, both driving vehicles.

 
From BBC news https://www.bbc.co.uk/news/live/c9qeq08g70yt


Edit to clarify quoting - Grahame