An authority may not condemn or purchase real property of a rapid transit authority operating pursuant to Chapter 451 that was confirmed before July 1, 1985, and in which the principal municipality has a population of less than 1.3 million , unless the authority has entered into a written agreement with the rapid transit authority specifying the terms and conditions under which the condemnation or the purchase of the real property will take place.
Tex. Transp. Code § 370.192