Bridges often have symbolic weight when they appear in literature. There are two bridges in The Mayor of Casterbridge, one brick and one stone. Both bridges attract people who feel like "failures" (32.4). Sometimes these people try to throw themselves into the water below, but mostly they just like to lean against the rail, stare down at the water, and feel dejected.
Why would a bridge be an appropriate place for people who feel like "failures" to congregate? What might the bridge symbolize?
Bridges represent connection. They literally connect different places, and metaphorically they suggest the connection of ideas. So standing on a bridge might suggest that a person is stuck at some "in-between" stage – between jobs, between relationships, or unable to make a decision. Lingering on a bridge could symbolize stagnation: the person is standing still while the water below and the passersby on the bridge keep moving.