Preowned. SingleTrak Pack from Ruffwear is a sleek and low-profile day backpack for dogs. The Ruffwear Singletrak Pack is a sleek, low profile hydration pack. Comes with one 0.6-liter collapsible hydration bladder. Features an added assistance handle to make it easy to help your dog up and over obstacles. It also includes stash pockets, coated zippers. SIZE: M: 27 - 32", (69 - 81 cm) Girth