He holds on to the railing just long enough to make sure he's got his balance, then turns and lets go.

The beam's just barely wider than his shoe. Bucky holds his arms out a little, one to each side, and starts cat-footing his way across. He keeps his eyes fixed on his goal, only looking down in brief glimpses to check his footing -- just enough to see the explosion blooming beneath him like the pits of hell.

Metal groans, and the beam shivers; he sways with it but catches himself, and starts moving faster.