The circle of stones in the path might make it a mill of some sort

Its a safety zone from falling coconuts.
When it gets windy, everyone runs there for cover.

Grist Mill

My first thought was Lich gate, but I think it would be fancier and not so long and there would be a wall.

Correct!!

Well, there are no coconuts here, but Classicman has it!

The path goes under an old, disused railway viaduct with a slightly crumbly exterior.

The roof is stronger than it looks, made of logs sawn in half.

Pic in a second when I find it....!

From the other side first.

way to go classic!

I wonder if any engineering went into designing that roof, or if they just decided to go with the heaviest timbers they happened to have available. Like how do they know how massive the chunks will be that happen to fall down? There's enough weight up there to flatten this structure like a pancake if it all comes down at once. So they are making an assumption that it will come down in pieces a little at a time. How did they decide that the pieces would be a certain size?

I was sort of kidding. (lot more than a little)
We were directed to a couple of those when in Hawaii.

I think the viaduct itself is pretty secure as a structure (I walk over it fairly regularly and it hasn't collaped under my weight!) and there doesn't seem to be any big chunks missing, just some crumbly bits here and there.

Perhaps, they were worried about lumps of coal falling from the railway.

Or spitters.

Or rocks from the railbed, and maybe ice in winter.

nuts, bolts, spikes, and plates.

Or hobos.

Rhianne that's a gorgeous photo!

Maybe in case it starts