time-lapse video on YouTube. The bowstring truss is just the public face of the bridge: the other side is hard up against the old railway bridge and it's just a flat girder, albeit massive. The aggressively muscular truss side is designed for effect, and not everyone will love it.
It comes as a surprise to find that no trains will use the bridge until work on London Bridge station is complete, some years hence. When everything is in place, though, this will be a vital part of the new Crossrail link currently under construction across central London.