(a) A transit department is a separate governmental unit for purposes of Chapter 101, Civil Practice and Remedies Code, and operations of a transit department are essential governmental functions and not proprietary functions for all purposes, including the purposes of that chapter.
(b) This chapter does not create or confer any governmental immunity or limitation of liability on any entity that is not a governmental unit, governmental entity or authority, or public agency or a subdivision of one of those persons. In this subsection "governmental unit" has the meaning assigned by Section 101.001, Civil Practice and Remedies Code.