This new contemporary boxed meal dining option sounds more like being served on an airline than in a restaurant. If airline staff aren't tipped for delivering a meal is there a reason why train staff should be?
Do passengers generally tip the attendant when getting their boxed meals on the Portland section of the Empire Builder? I have traveled both ways only once and I do not remember. This meal service may be similar if the passenger chooses to eat in the sleeper lounge as opposed to having the meal brought to her or his room.
My last trip on #28, our meals (quite good BTW) were delivered to our roomette. As we only traveled to Spokane from Portland, we probably only tipped about "normal" for a less than one night trip. I don't recall adding extra to what I tipped because of the meal delivery.