Did that once with a club in the early 90's. Lot of detours due to some washed-out track, plus gated and caved tunnel. I remember the biggest threat on that side trail as being large stands of teddy-bear cholla cactus, which I avoided like it was radioactive, but almost everyone else got these "funny fuzzy balls" stuck thru the tires/clothes/skin. A massive tube-replacement party ensued, then the final tunnel at Dos Cabezos being gated, we had to squeeze the bikes and ourselves though a passage dug under the gate.
Are you sure the line is abandoned? I would have though it would have been blocked off again, then. And be aware that the owners in the 90's and the state park rangers did try and catch and ticket trespassers, since that's a legal wilderness you're in if you stray one mm off the tracks.