Looking at the available video the bridge damage looks to be more from charges beneath the span that was dropped. The time of day it was detonated suggests it was the Ukrainians and not the FSB as that was a time of expected low traffic. If you look at the staged 'terrorism' that happened as a leadup to the second Chechen war, the FSB clearly aimed for as many casualties as possible. I'd expect the same in this situation.

But the fact that it was the road span, not the rail suggests it was not Ukrainians as the rail link is logistically much more important than the road (and just one side of the road at that). Also, it looks like two spans were dropped. Why drop two on the same side rather than a single one on each side and bring road traffic to a complete halt?

Of course, it is possible that it was a massive truck bomb, although the photos presented of the truck don't match up and explosions tend to go upwards.

It's a puzzle.