The road bridge explosion took out a train hauling fuel tankers on the parallel span.
Ukrainian Intelligence suggests it might have been an inside job, that Putin's Russian opponents might have been behind it.
There is evidence it was a large bomb in a truck, which implies either a suicide bomber was driving it, or the bomb was surreptitiously placed in with explosive cargo by saboteurs.
The Russians are all upset how their enemies are attacking "civilian infrastructure", You've got to hand it to these guys, they have solid brass cojones.