Wouldn't it be easier for them to completely replace them with Siemens? If not, they will have to permanently use Horizon sets. I doubt they will order more Talgos, as the Wisconsin ones have been sitting there for ages and no moves.
Here's the easiest way to break it down: WSDOT owned three Talgo Series VI trainsets, Amtrak owned two Talgo Series VI trainsets, and ODOT owns two Talgo Series 8 trainsets.
One of the WSDOT owned Talgo Series VI trainsets was destroyed in the 2017 derailment.
WSDOT was awarded a $37.5 million federal grant to cover 50% of the cost of purchasing new passenger rail cars to replace the three Talgo Series VI trainsets.
WSDOT, long ago, committed to purchase the same equipment as the Amfleet I replacement... so that money will be used to purchase three trainsets from Siemens.
It's also my understanding that WSDOT also received insurance proceeds from Amtrak (compensation for wrecking equipment owned by WSDOT). A big chunk of that money was used to purchase Charger #1408, but there should also be money left over to further offset the cost of the new equipment.
Presumably, two Amtrak-owned Siemens trainsets would also be dedicated to the Cascades.
That just leaves the ODOT-owned Talgo 8 trainsets. They are only eight years old, and they are fully FRA-compliant... so I doubt that ODOT wants to get rid of them... but for as long as they have them... Talgo demands that they do the maintenance on them... which could be a lot more expensive with just two trainsets to service, instead of seven.
Oh, it's also worth mentioning that Amtrak committed to WSDOT that the very first Amfleet I replacement trainsets to come off the line would go to the Cascades.