Me. Rev. Stat. tit. 23, § 5123

Section 5122 shall not prevent railroad corporations from establishing necessary rules and regulations for the cancellation of tickets and exchange of partially used tickets, but the rules and regulations shall be publicly posted at each ticket office and on all passenger trains and, when practicable, printed upon the tickets. Any ticket or check given in exchange for the unused portion of a partially used ticket continues in force for the full term of the original ticket, as provided in section 5122.