Curious, as I'm struggling to see what PP you did that would have caused this.
Looks like a straight crop on either side would have covered much of what you wanted to bring the subjects into prominence.
Actually, I was also originally puzzled why the straight lines of the railings seemed ok. Now it's a little clearer!
But can you tell us what s/w you used and what you did?
The Fuji 18-55 has some barrel distortion at the wide end, but nothing remotely like this.
