Tex. Tax Code § 162.356

(a) The tax imposed by this subchapter does not apply to compressed natural gas or liquefied natural gas delivered into the fuel supply tank of:

(1) a motor vehicle operated exclusively by the United States, provided that the exemption does not apply with respect to fuel delivered into the fuel supply tank of a motor vehicle of a person operating under a contract with the United States;

(2) a motor vehicle operated exclusively by a public school district in this state;

(3) a motor vehicle operated exclusively by a commercial transportation company or a metropolitan rapid transit authority operating under Chapter 451, Transportation Code, that provides public school transportation services to a school district under Section 34.008, Education Code, and that uses the fuel only to provide those services;

(4) a motor vehicle operated exclusively by a volunteer fire department in this state;

(5) a motor vehicle operated exclusively by a municipality or county in this state;

(6) a motor vehicle operated exclusively by a nonprofit electric cooperative corporation organized under Chapter 161, Utilities Code;

(7) a motor vehicle operated exclusively by a nonprofit telephone cooperative corporation organized under Chapter 162, Utilities Code;

(8) a motor vehicle that is not registered for use on the public highways of this state and that is used exclusively off-highway;

(9) a motor vehicle operated exclusively by a nonprofit entity that is organized for the sole purpose of and engages exclusively in providing emergency medical services and that uses the fuel exclusively to provide emergency medical services, including rescue and ambulance services;

(10) off-highway equipment, a stationary engine, a motorboat, an aircraft, equipment used solely for servicing aircraft and used exclusively off-highway, a locomotive, or any device other than a motor vehicle operated or intended to be operated on the public highways; or

(11) except as provided by Subsection (b), a motor vehicle:

(A) used to provide the services of a transit company, including a metropolitan rapid transit authority under Chapter 451, Transportation Code, or a regional transportation authority under Chapter 452, Transportation Code; and

(B) operated by a person who on January 1, 2015, paid tax on compressed natural gas or liquefied natural gas as provided by Section 162.312, as that section existed on that date.

(b) The exemption provided by Subsection (a)(11) does not apply to compressed natural gas or liquefied natural gas delivered into the fuel supply tank of a motor vehicle from a refueling facility accessible to motor vehicles other than:

(1) motor vehicles described by Subsection (a)(11)(A); or

(2) motor vehicles exempt from the tax imposed by this subchapter under another provision of Subsection (a), if the:

(A) refueling facility is operated by a metropolitan rapid transit authority operating under Chapter 451, Transportation Code, in which the principal municipality has a population of less than 320,000; and

(B) use of the refueling facility by those vehicles is limited to emergencies and other exigent circumstances as provided by an interlocal agreement under Chapter 791, Government Code.