Actually, it's pretty permanent and has been for several years, now. It's not specifically a Coast Starlight car but rather just a generic Superliner coach that I believe Amtrak California does lease from Amtrak for extra space. It's usually positioned immediately behind the business-class car.

I've always taken the Superliner car if it's open (roomier seats, plus it tends not to fill up as quickly because the downstairs door is manually operated and thus doesn't open at stations), but the one downfall always was that it lacked wifi (so I'd have to use up my own data while working en-route). I was pleased to discover on a recent trip on the Surfliner that they've installed an access point inside the car.