Seems like I'm always seeing folks looking for locations involving RR bridges for shoots, and being a train nut I am pretty familiar with the existing bridges in the area. However most of them are in use and therefore illegal to trespass upon to say nothing of very very dangerous to be on.
That said, I understand the desire is always going to be there so I've been trying to find truly abandoned bridges that folks could use. Several others have posted a few, like the one under US 59 near downtown Houston. I think someone here posted a link to bridgehunter.coim awhile back, and I was perusing it recently and came across on FM1093 bwetween Willis and Simonton. The tracks are not only abandoned but removed so as far as railroad traffic goes it's pretty safe. The gaping holes in the decking are another matter... But it may be useful, so here's a link:
Bridgehunter.com | Wallis Railroad Skew Through Truss
can't vouch for ease of access or safety, I'll leave that to you. But you won't be bothered by trains.