Check it with a magnet, many times it's a copper coated steel or copper/steel alloy. Lot's cheaper.
I built a 10 ft span foot bridge over my creek with 6 of them side by side, decked with treated 2 x 12's.
Leaching Creosote can be toxic to live stock, and people.

Don't forget the osprey's leftovers...we went down to Charleston, SC a free years ago and went to the USS Yorktown and an osprey had a nest up above the bridge. You could smell the fish pretty strong and a few kills were laying on the deck below.

As for the baldies...our county conservation club has placed two nest poles which have to be at least 50' above ground. We have eagles near both boxes but only once has had a sniff of a bird using it.

Need a bridge anywhere?

We were given some that had to be pulled, Friend cut decking size boards out of them. Nasty to handle the creosote /oils But boy they bead up water. i would not want them too near my house but to keep out of mud at a camp.... plus he said they cut super east on the bandmill

We made a elevated coyote hunting shack for our bait pile with insulation and heat.

I built a big swing set with 2 swings from poles. It also serves as a Tarzan rope and a zip line platform
I put it right beside my pond

I would doubt most saw mills will touch them. To much metal and it only take a little to dull a mill blade. There are probably metals in them that the detector won't pick up so I think most mills would feed you beans. Now, if you have a buddy with a home mill or use a chain saw attachment, you could make lumber.

They can be used to build a rudimentary birth control device.

Make sure to always wear a helmet, eye goggles, a respirator, a hazmat suit, steel toe boots, and shoulder length rubber gauntlets.

Actually, I wouldn't mind getting a few myself. I want to build a dock when I make the pond bigger and maybe a greenhouse overhanging the pond. They will be perfect for the main posts. I'll have to look into it in my area when I'm ready.