Can't remember why I searched this, but the bottom lines seem to be: 1) rail service sucks (trains as much as 70%+ late on some routes, average on the worst routes of 40+ hours late); and 2) USPS management and staff suck worse (missing cutoff times for trains, not managing the contractors closely enough (or at all in some cases), not documenting things, etc.). Documentation of course golden for an auditor so USPS staff not religiously filing those forms is a major problem (they're trying to figure out the recipe that was used in the cooking). Anyway, the underlying message seems to be: don't stop using those Highway Contract Routes just yet for surface mail.

[www.uspsoig.gov]

PS: yes, I know, it's a 2022 report. I just saw it so it's new to me. Might be new to at least one other person. ;)
PS2: the report of course doesn't identify the contractors. But the lanes used would be consistent with #1 = BNSF and #2 = CSX.