I think what you are experiencing is a Moire pattern effect due to the angle and distance between the rails. 
This disappears in the larger image because the distances have changed.  A larger file will help though I doubt it is any problem with the printed image just an unfortunate limitation of small sizes on a pc screen.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html