Because rail is quite non line of sight and quite infrastructure defined pathing.

The automatic coordination system hit a fail condition; they neglected the fallback protocols that had rules for spacing determined to work reliably without the computer keeping real time supervision; they crashed.

This is in the way more like the Washington D.C. helicopter plane mid-air where some normalization of deviance got people to ignore automated protection systems.

If most involved had known the fallback protocols they would have refused to move/continue while the protocols were utterly neglected. At least with a robust safety culture active, ofc.