I was wondering the same thing when I saw the cover, but I thought the ties were wooden, not concrete.
I even said to myself, where was this taken...Antonito or something
And the trees and ballast doesn't look like any D&RGW dual gauge areas.
Looking inside there was no mention where the photo was taken
And if this was a bridge track or other section that typically has guard rails, there would usually be two guard rails on standard gauge track.
It might not even be from the US, although the article is about saving US passenger trains. It could even be a file/stock photo from who knows where.
So like you, I'm stumped.