Based on Business Class seat counts from railsforless.us, 516-519 is running Talgos next week, at least Sunday (22 open BC seats), Monday (31 BC seats), Friday (17 BC seats) and Saturday (24 BC seats). Tuesday (2 BC seats), Wednesday (2 BC seats), and Thursday (4 BC seats) are questionable. Those represents a nearly sold out BC. That’s easy with the Horizons, I think they only have 18 BC seats, harder with the Talgos with much higher BC capacity.
I've been watching it recently because I am traveling on it on Sunday, 11/3 and will do a coupon upgrade to BC if it's a Talgo. BC hasn't been generally getting that close to sold out recently, which makes me think that might indicate Horizons midweek next week. Or it just may mean BC sales have been good.
EDIT: rechecked the prices and they're at lower buckets than the top $111 bucket for those midweek trains. Since, for BC, railsforless.us shows only the inventory in the current bucket that means it isn't the remaining inventory. It actually means sales are low those days and probably does not indicate Horizons are running midweek, given Talgos are running other days.
I don't have any clue to the logic behind the assignments of the two ODOT Talgo sets on the Cascades. Based on physical observation at EVR, 516-519 were running Talgos in July, but they got reassigned and 516-519 were running Horizons in August. Recently it's been back to Talgos 7 days a week. While I haven't seen a pattern varying day by day, they've running consistently either one or the other, but, as I said, Talgo Cascades assignments are a mystery.