Google Earth shows a really nice bird's eye view of the trails, check it out. You can find it by looking in the SW corner of the 360 and Trinity interchange.
To get there from 360, take the Trinity exit and go west. Turn south on Tarrant Main St. Cross the RR tracks and your there - Tarrant Main dead-ends into the parking area.
I'm not exactly sure how "legal" it is to ride there, however, it seems to be an "accepted riding area". I think the problem is liability. From what I understand, that area has recently been incorporated by the city of Ft. Worth, so folks riding out there may pose as a liability issue for the city if someone gets hurt.
Hope this helps,
Brad