34 Tex. Admin. Code § 9.4005
The value of railroad stock will be allocated to this state in the proportion of the total market value of the rolling stock that fairly reflects its uses in this state during the preceding tax year by ascertaining the average number of cars found to be habitually within the borders of the state or the average amount of the property habitually used or employed in the state; or, when such method will produce a more just, fair, equitable, and lawful result, by ascertaining the proportion of the total distance in miles of main line track, branch line tract, and side, yard, and spur track located in the state which the railroad company operates in comparison with the total distance in miles of such tract which the railroad company operates.
34 Tex. Admin. Code § 9.4005