Gnashbar has a backpack with pannier hooks under the back pad to hang it from a rack.

Ortlieb has excellent panniers that can take their accesory back pack kit to clic on the pannier

They keep stuff inside Dry .. you can buy just one, aka the Shopping Bag It uses a Waterproof Zipper

With a Brompton Folding bike they have a large bag It goes on the front
ride to the train station Fold the bike up and Get on the train with it .
sling the front bag over your shoulder..
thats what they were made to do..