49 CFR 260.39: Maintenance standards.

(a) When the proceeds of a direct loan or an obligation guaranteed by the Administrator under this part are, or were, used to acquire, rehabilitate, improve or construct track, roadbed, and related structures, Borrower shall, as long as any portion of the principal or interest of such obligation remains unpaid, maintain such facilities in at least the highest track class, as defined by FRA Track Safety Standards in part 213 of this chapter, specified in the Application at which the rehabilitated, improved, acquired, or constructed track is to be operated upon completion of the project.

(b) When the proceeds of a direct loan or an obligation guaranteed by the Administrator under this part are, or were, used for equipment or facilities, the Borrower shall, during the period in which any portion of the principal or interest in such obligation remains unpaid, maintain such equipment or facilities in a manner consistent with sound engineering and maintenance practices and in a condition that will permit the level of use that existed upon completion of the acquisition, rehabilitation, improvement or construction of such equipment or facilities.