Thanks to comments from @mcadv and others in this forum, as otherwise I wouldn't have known that this connection (regional Renfe Feve) even exists.

Used our Passes last Friday on the 71872 13:31 Bilbao-Santander service. We had the time and continuous Passes so why not…

Times given on https://www.renfe.com/es/en/suburban/cercanias-feve/timetables were accurate.

On the Renfe home page planner https://www.renfe.com/es/en the train came up but with incorrect times (30 minutes out for some reason).

Couldn't find the train in any planner apps.

Departing from Bilbao La Concordia, next to Bilbao Abando but a separate building. A nice peaceful station. Showed our active Passes at the ticket counter and they gave us tickets to use to get through the closed gates. I'd asked the day before just to make sure and the staff confirmed both times this is how they do it.

Views were better than we'd been anticipating, nice suprises pretty much all the way.

Conductor was happy enough seeing the printed tickets and didn't seem too bothered about the Pass, though we had added the journey manually and showed him the Pass.

Small train with a toilet and about 40 passengers, which got full towards the final stops in Santander.