My front panniers, and rear panniers, snap together, but even so, I used a long cam-buckle strap around each. So like stringing a square package, around once and then crossing the two strap ends to wrap in the other direction (90 degrees different), then engaged on the cam buckle, cinch tight, done. So 4 panniers equals 2 checked (or carryon) bags. No need to repack, and straps pack a lot smaller than another large bag. I forget the size limits on carryon. On the train trip I was on, tons of luggage space in racks at end of cars, and overheads.

For a plane trip, I might put the panniers in duffles, to protect against abrasion, and theft. But trainside checked baggage is not subjected to the abuse of airline baggage handling systems and personnel. More likely, my folding bike in a bag or box will have clothes in bags and empty panniers packed around it, and some stuff brought as carryon.