If I may add my uninformed analysis...
While I've heard nothing to substantiate this, I'm fairly convinced the 'Keag line has not seen its last 'real' train. The line is fairly obviously of strategic value when looking at the current map of Maine's railroads. However, it would seem that giving away some business (as they most certainly have since opting to use CM&Q), is more than made up for by the reduction in costs of operating the line to 'Keag. They seem to be in a leaning out and cost controlling phase. When that changes I would imagine that a quick way to grow the revenue would be to return to 'Keag and steal back some of the business they've lost to the Moosehead, as well as grab a share of new markets (which might be the trigger for said return).
And contrary to assertions elsewhere here, I'm convinced the service provided by the NBSR-CMQ-ST routing is no more expedient than the old NBSR-ST routing. Track speeds may be higher, but the addition of a second interchange, and corresponding terminal delay wipes away any gains made in transit time. So the whole line "its better for customers and everyone involved" is false. The simple fact is, it saves Pan Am money while they work to control their costs. It will serve its purpose until either CM&Q gets greedy and plays rate games, or new potential traffic develops that would provide enough revenue to make termination of the haulage deal worth it.