That aside I've seen the video circulating showing the explosion...

That aside I've seen the video circulating showing the explosion from another angle. There is a small water disturbance under the span immediately before a much larger fiery explosion which balloons up over the side of the bridge, just after the truck has passed and blows fiery debris across both lanes. My thoughts are this was a small detonation used to trigger a larger one, and the small underwater explosion occurred maybe a second or so before the second bigger one. Or possibly there were two large charges in different places detonated sequentially to bring down the span.

Definitely it wasn't a truck blowing up. The spans are broken and shifted off the supports, but there's no sign of explosive damage to the surface. And the explosion didn't damage the eastern section of the bridge which pretty much rules out the railway explosion as being the cause. It would have taken out the eastern lanes first.

Simple logic tells you there were multiple explosions, both on the train and the bridge. And it was no truck bomb, but a detonation (or more likely two) UNDER the western span that brought down several sections.