In a nutshell:
The next stage in Arkel’s trunk bag revolution, the Tailrider Expedition offers features customers love from the original model - it's sleek, aerodynamic, functional and big; everything a trunk bag should be- but now we’ve added fully waterproof, roll-top, drop-down side panniers. These taped and sealed dry-bag panniers easily store in the side pockets of the Tailrider Expedition or can be completely detached. The Tailrider Expedition includes a carry handle. Design constraints reduced the top bellow storage space slightly but expanded side pockets make up the difference. An optional shoulder strap is not included - it can be ordered separately.
For when you want to take the trunk bag experience to the next level, further, longer and with more gear.