Returning to the attack on the bridge, the "truck bomb" idea doesn't hold water.
The direction of the explosion, the flying sparks and debris, and the visible damage after the event all rule out a truck or road vehicle bomb.
The road deck sections are much weaker for upward forces. As they are designed to cope with downward forces. IMHO, it was explosions from below or from below and to the side that caused this damage to the road decks (three sections have been damaged). The resulting shower of shrapnel pierced multiple fuel/oil tanker wagons, igniting the multiple fuel/oil leaks hence the multiple fires.
I see reports on Twitter that the Russian rail authority claims that trains are now running over the rail bridge.
Keep in mind that the railway is double track (two lines). If the tanker (fuel/oil) train has not derailed or is otherwise foul of the adjacent line, it’s possible to run trains on the other line.
However, the structure still needs to be inspected and assessed to determine how badly it is damaged and if any weight or speed restrictions are needed. Assuming that it can actually be used safely.
The high temperatures will have changed the properties of the steel used for the structural elements of the bridge sections. This will have weakened it.
Of course, Russia may not care about safety…