Some of this has been hashed out here in the immediate aftermath of the incident, but this Seconds From Disaster-style documentary covers all the bases and while it pre-dates the Metro-North derailment, the similarities are quite apparent:
Not mentioned in the documentary is that after the disaster, RENFE installed additional transponders toward the end of the high-speed segment so that the ERTMS system could enforce speed reductions before trains left the high-speed trackage.
---PCJ
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