Riverside Center, the formerly-stalled five tower complex coming to the far Upper West Side, is finally on the rise. Well, at least one of the site's towers is rising. That would be 21 West End Avenue, SLCE's contribution, which will be a 43-story, 616-rental building with a public school in its base. Field Condition recently scoped out the site, and reports that construction on 21 West End Avenue has surpassed the half way mark, but the project's other towers aren't so far along. FC speculates that 1 West End Avenue, Silverstein and Elad's rather pricey "dual-entrance" building, will be the next to rise above grade. When complete, the megaproject will bring 2,500 apartments to a site formerly used as a rail yard and parking lot.