Tex. Transportation Code § 454.029

(a) While bonds or notes that are payable from and secured by a pledge of the revenue of a mass transportation system are outstanding, the municipality that issued the bonds or notes may from time to time issue other bonds or notes for the purpose of:

(1) extending, improving, or both extending and improving the system; or

(2) acquiring another mass transportation system.

(b) Bonds or notes issued under Subsection (a) constitute a lien on the revenue, in the order of their issuance, inferior to the liens securing all issues and series of bonds or notes previously issued.

(c) Notwithstanding Subsection (b), a municipality may:

(1) adopt an ordinance or execute and issue a deed of trust, trust indenture, or similar instrument that provides for the subsequent issuance of additional bonds or notes on a parity with the previously issued bonds or notes; and

(2) authorize, issue, and sell additional bonds or notes, from time to time and in different series, payable from the revenue of the mass transportation system and the revenue of any additional sources, on a parity with the bonds or notes previously issued and secured by liens on the transportation system that are on a parity with the lien securing the previously issued bonds or notes, subject to the conditions of the ordinance or instrument described by Subdivision (1).