Yep, what they say above.
This brings you full circle. Back to RR ties and what is there now. Consider evaluating what is in place. Replace the bad ones and plank over.
It's obvious from your initial pics that some of those need replacing. I would take a nail(in a comfortable handle) or an awl one size smaller than the nails you will be using to build. Go over each RR tie that you check on the bridge or that you buy. If good/new 4x4's are strong enough, then 6x6' that have been used and still solid should be good to. Couple this with the fact that if you keep the spacing the same as what's on there, and the planking on top of it, it should be as good as recommended or possibly better.
One way you could test them, but at a lot of extra work. This would test them structurally, which is a bit different soundness ( nail test). Both things are needed for your application. Place some beams 8' apart on the ground. Lay out the RR ties across them (not fastening)making a solid enough deck to drive on, but enough space so that each one has some wiggle room, just RR ties no planking. Drive your tractor across. Any that you even hear cracking or bending more than the average, replace. Roll the ties over 180 degrees and repeat. Possibly worth the effort....
And as I said, work safely. Some of those old ones that are on there can give out it a blink of an eye.
