Mass. Gen. Laws ch. 31 § 65
Original and promotional appointments in the Massachusetts bay transportation authority police force shall be made only after competitive examination.
An examination for a promotional appointment for any title in any such force shall be open only to permanent employees in the next lower title in such force, except that if the number of such employees, or the number of applicants for the examination, or the number of applicants presenting themselves for examination is less than four, the examination shall be opened to permanent employees in the next lower titles in succession in such force until either four such employees have applied and presented themselves for examination or until the examination is open to all permanent employees in lower titles in such force; provided, that no such examination shall be open to any person who has not been employed as a permanent employee for at least one year in such force in the lower title or titles to which the examination is open, and provided, further, that no such examination for the first title above the lowest title in such force shall be open to any person who has not been employed as a permanent employee in such force for at least three years in such lowest title.
All promotional appointments to fill positions in such forces shall be made on a full-time basis. No such promotional appointments shall be made on a permanent intermittent basis.
An eligible list established as a result of any examination shall not be used for an original or promotional appointment to any position in any force pursuant to this section unless the announcement of such examination identified such position as one to be filled from such list.
Notwithstanding the provisions of any law or rule to the contrary, a member of any such force who has served as such for twenty-five years and who passes an examination for promotional appointment in such force shall have preference in promotion equal to that provided to veterans under the civil service rules.
Following his original appointment as a permanent full-time police officer in the Massachusetts bay transportation authority police force, a person shall actually perform the duties of such position on a full-time basis for a probationary period of twelve months before he shall be considered a full-time tenured employee in such position. The administrator, with the approval of the commission, may establish procedures to ensure the evaluation by appointing authorities, prior to the end of such probationary period, of the performance of persons appointed as police officers in such force.
Mass. Gen. Laws ch. 31, § 65