VALLEJO — Cyclists and pedestrians can use a brand-new segment of the Napa Valley Vine Trail segment – a portion of the railway that is neither in the Napa Valley nor lined with vines.

Just three-quarters of a mile long, the oak-and-vineyard-dotted road features a rusting, one-lane truss bridge over the Napa River that’s prone to flooding. (A bright-yellow traffic sign reads ...