It was a truck bomb. There's actual security cam footage on Twitter showing it explodng. And this explosion also blew up the train tank cars. Unfortunately 3 civilians were killed.
The worse part is Russia has hi tech inspection systems on the bridge that are supposed to detect and stop events like this, so it's yet another failure. They say the rail line will be repaired later today, but I'm sure the bridge's structural integrity was compromised by that much fuel burning. The walkways along the bridge were melted and this fire went all the way down inside the bridge deck.
And Ukraine will be making stamps to commemorate the event. :P