If the revenue of a mass transportation system is encumbered under this chapter, the expenses of operation and maintenance, including all salaries, labor, materials, interest, repairs, and extensions necessary to render efficient service and each proper item of expense, are a first lien against the revenues. The expense of an extension may be a lien prior to an existing lien only if:
(1) the governing body of the municipality considers the extension necessary to keep the system in operation and render adequate service to the municipality and its inhabitants; or
(2) the extension is necessary to meet a condition that would otherwise impair the original securities.