We used this one from amazon, RedCamp 96L. Perfectly fits two large rear panniers, two fronts, aaaaand the handlebar bag, aaaaaaaand the tent. We had 2 duffels for two people. Worked perfectly. Then when we got to destination just stow the duffels in one of the panniers and off you go! The duffels are pretty light and packable. When you fill them with panniers, its so easy to carry all the stuff in one bag.

We have used this method for flying to and from Europe, taking a few busses with our bikes, and SurfLiner train when on the West Coast. Should work with Amtrak too!

https://www.amazon.com/REDCAMP-Duffle-Lightweight-WaterProof-Foldable/dp/B07FTD88K4/ref=sr_1_12?keywords=large+duffel+bags+for+traveling&qid=1649600646&sprefix=large+duf%2Caps%2C440&sr=8-12

Enjoy your trip!!