Passenger speeds on the P&L could have been higher than freight. From Marshall to Oakesdale could be up to 49 for freight without much effort or signals. See an old NP timetable for Budd car run times. I recall a study that had BN running empty grain and coal trains over the CSP & P&L routing like Stampede is being used now. With current motive power and the fact the power always stays with the train it would be very do-able. (Also running times could actually beat current routing -- no meets!!)
UP route would be better, but UP doesn't want it at all. For Amtrak, remember, they wanted to pick up Pullman and Moscow for the passenger traffic -- students to ski, and students from Wazzu headed west for breaks. I remember Amtrak at the time was willing to stage a coach at Pullman for such break traffic.