Well, it looks like it could hold a lot of weight, seeing there's very heavy equipment moving around on it.
They're doing so well that they have test runs on the bridge already!
The other possibility of course is that they’re driving plant and equipment over it during construction on just the plain structure, and that there will be a more ‘decorative’ top layer added once they no longer need to drive all over it?
That's my bet. If it's just 2x4/2x6's, I'm guessing it'll look similar to Lost Continent's bridge when it's all said and done. A couple coats of varnish and some fake wear and tear, it'll do the trick.