[Figment]

Waiting to see what investigation reveals. Rail car couplings aren’t small and tend to hold well although they are quite easy to disengage with a simple handle

[Paladin2]

The only two photos of the rear of the trailing locomotive I’ve seen do not include any info about the state of the rear coupler.

[Paladin2]

Pic of full view of rear of rear locomotive here:

http://www.gettyimages.in/detail/news-photo/train-from-the-mma-railway-is-viewed-as-it-was-stopped-by-news-photo/173183169

[Paladin2]

"Waiting to see what investigation reveals."

http://www.tsb.gc.ca/eng/enquetes-investigations/rail/2013/R13D0054/R13D0054.asp

[Paladin2]

Here's a closeup pic of part of the Lac Megantic rail yard:

I'd suggest that the box car on its side with fire oxidation of the aft end was coupled to the other three tank cars in the pic. It was the "buffer" car attached to the rear of the last locomotive. Its position wrt the inside track indicates that it was accurately following the locomotive directly in front of it, but was rolled over by rollovers of tanks cars somewhere to its rear.