Under what authority do you know the ties were 90 years old, that the ties or rails had failed, that the cars were being moved "too quickly"?

With all due respect you are part of the problem, not part of the solution.  Making raw assumptions without any professional knowledge.  How do you know the crews were intentionally moving the tankcar "quickly hidden from public view" for the sake of covering up evidence?  How do you know that the "Antiquated track was rapidly being scrapped" was still "evidence"?

Believe it or not, this amounted to the equivalent of a fender bender.  Yes, the cars were larger.  But that's it.  FRA likely sent an inspector, looked at it, and gave UP the go-ahead to clean it up.  It's like a minor wreck where the police just say "share insurance information and clear the road."  Was there any chemical release - was the tankcar even loaded?  If it were, you can be assured a much larger emergency response would have shown up.