The problem with this approach is that you forego any revenue that might be obtained from passengers riding between Boston and stops in New Hampshire, while retaining 100% of the costs. This increases the losses, which increases the subsidy requirement that Maine must pay.The solution to this, is to make all stops south of Maine as discharge only southbound, and receive only northbound, if those states don't contribute. That way Maine passengers wanting those stops would still benefit. ,Another way would be to charge a higher fare over those segments, perhaps even equal to the first stop in Maine. If it is found that the local out of state traffic benefits the train, they could just keep as is.
So you will almost always have to keep it as-is, which really eliminates the practicality of using that as a way to get intermediate states to pay for it.