MM&A train derailment on St Lawrence River bridge

By now, it's well known that a loaded 5 well double stack car on a Maine to Montreal MM&A freight was apparently blown off the tracks on CP's St Lawrence River bridge outside of Montreal. My question is from where does this double-stack traffic originate? Is it from St John on the New Brunswick Southern or does it originate on MM&A? I knew MM&A hauls piggyback for Sunbury but I was not aware that doublestack container traffic was also in the mix.

Thanks.

According to the Trains Magazine Newsline, these containers were all empties. Somewhat confusingly states that the well cars were loaded with empty containers. Not sure if this is a return routing and they were loaded in the opposite direction.