2-Segal sets and the Waldhausen construction

It is known by results of Dyckerhoff-Kapranov and of Gálvez-Carrillo-Kock-Tonks that the output of the Waldhausen S
- -construction has a unital 2-Segal structure. Here, we prove that a certain S
- -functor defines an equivalence between the category of augmented stable double categories and the category of unital 2-Segal sets. The inverse equivalence is described explicitly by a path construction. We illustrate the equivalence for the known examples of partial monoids, cobordism categories with genus constraints and graph coalgebras.