Mass. Gen. Laws ch. 161B § 9
If in any year the commonwealth shall be called upon to pay any amount on account of the net cost of service of any regional transit authority except the Cape Cod Regional Transit Authority, unless said authority elects not to assess costs as provided in section nine A, the total amount of such net cost of service shall be assessed upon the cities and towns which are members of such authority in the proportion which the loss attributable to each route in each such city or town bears to the loss attributable to all such routes in all such cities and towns. The loss attributable to each such route in each such city or town shall be determined on the basis of the difference between the revenues collected from such route in such city or town and the cost of providing such route therein.
Such determination shall be made by such authority in accordance with sound accounting practice and guidelines developed in consultation with the department. Amounts assessed under this section shall be the most recently audited regional transit authority assessment available on January 1 of each year and shall be used to calculate the upcoming fiscal year's estimated cherry sheet assessments.
Mass. Gen. Laws ch. 161B, § 9