Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 161, § 150

The cities and towns comprising a transportation area shall contribute to the discharge of its liabilities and obligations on the basis of one third part according to the single track street railway and the bus or other mass transportation route mileage running in or through their respective limits, one third part according to their population, and one third part according to their assessed valuation. The department shall establish the said basis at least once in three years.