Not a mesh bag, but a basic lightweight duffel. On my current trip, I've used it three times on a train to turn multiple panniers into one piece of luggage. I also used it one day on RAGBRAI when I used the baggage service.
I normally have it on my back rack with a small backpack inside. That is my food backpack and so depending on how much I get at the current section it will expand and contract.
