Alistair Heaton said:
More interested in what the effects of 8 railway tanker loads of burning fuel will do to that monocock beam section that is the structure of the railway.
Not to derail this topic anymore than it already has been (no pun intended), I certainly would not want to travel over it myself.
Having said that...
Unofficial consensus right now is 'something' exploded on top of the NW bound lanes (Russia->Crimea) of the east span causing mid-span failure which subsequently shifted multiple deck pieces from their foundation mounts.
Additionally, the west span containing the SE bound lanes (Crimea->Russia) experienced severe deformation of the structure holding up the east-most lane.
The freight train hauling petroleum and oil subsequently caught fire (did not explode) as it was showered with debris. In most of the early event photos you can see oil seeping down the trestle structure and burning across the various surfaces. There also were several utility lines suspended from the rail trestle, however their roll in this event, if any, is currently unknown.
As of October* 8th Russia has:
Re-opened the west lane of the west span to light 'civilian' traffic at spaced intervals.
Lorries are not permitted on the bridge. Approved shipments may move via ferry.
Initially all waterway travel around the bridge was suspended. Vetted cargo traffic has now resumed.
Full-speed rail service has resumed. (yes, less than 12 hours after the incident/fire)
This image shows the collapsed span from below. View is looking NW towards Crimea. Rail trestle is to the left.
Edit: Date correction.