"The crew received an alarm from a wayside defect detector shortly before the derailment indicating a mechanical problem, and then an emergency brake application was initiated.[30]"
I'd say the meat and potatoes of XATG's alleged responsibility starts around page 20 of this doc.
https://storage.courtlistener.com/recap/gov.uscourts.ohnd.294603/gov.uscourts.ohnd.294603.119.0_1.pdf
"However, railroads are limited in their ability to conduct a detailed inspection or take other action. They cannot, for example, test or certify a tank car’s safety-related mechanical functions—as car owners must do under 49 C.F.R. § 180.509 and other provisions—or load the hazardous materials to ensure proper stability and pressure for shipment—as chemical shippers must do under 49 C.F.R. § 173.22. It is the shipper that has the primary obligation to ensure that the railcar and equipment used are compatible with the lading."
Around page 9 you can read the full details of the various HBD alerts.
Around page 25 it gets into GATX's alleged culpability:
"For GATX Corporation’s Car 29, the tank car valve was replaced with
a Midland 720 valve without approval. Additionally, this Car’s PRD,
which had a 225 psi start, was replaced without approval with a PRD
with a 247.5 psi start. Finally, according to the Federal Railroad
Administration, Car 29 was never approved for vinyl chloride service,
which has a special regulatory provision."
"All three vinyl chloride tank cars owned by Oxy Vinyls—Cars 27, 28,
and 53—have missing or incorrect information on their AAR Form 4-
2. For example, Car 27 has modifications that were not properly
documented or approved, including changes to two component parts
that modified the material from carbon steel to stainless steel"
I think the document lays out pretty well what the car owner responsibilities are and what the shippers responsibilities are.
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