Amtrak bike boxes are $15. They are huge. Just take off your pedals, turn your bars and put it in. We put all four of the empty panniers in the box. We have an ultra light duffle to hold gear that goes in the panniers.
That large yellow bag in the foreground weighs about 6 ounces and will hold everything that fits in my panniers.
All we have to carry on the trains are the rack packs, handle bar bags and the duffle bags.