After Years of Halting Steps, A Transit Hub Begins to Take Shape.

For a moment, Uday R. Durg, who directs Lower Manhattan projects for the Metropolitan Transportation Authority's capital construction unit, looks out of place in his hard hat and reflective orange safety vest. He is standing just a few yards from a construction zone at the Fulton Street Transit Center, but he is surrounded by ordinary straphangers in ordinary street clothes. Whether hurrying through the handsome new straightaway that has replaced the maze of switchback ramps along the A and C lines or standing on the widened, ornamented and newly opened southbound platform on the R line, the traveling public has begun to occupy completed portions of the sprawling complex. ''For the first time,'' Mr. Durg said, ''people are walking through the transit center.'' [ABSTRACT FROM AUTHOR]