I assume that was crude oil on the train. How does crude oil “explode?”
I believe there individual tanks that ruptured “explosively” due to build up of pressure without an initial rupture while setting in a pool fire from other tanks.
The story linked below is the best timeline of events I have found.
http://www.edmontonjournal.com/news/M%C3%A9gantic+fire+timeline/8626739/story.html