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It looks like it is built from sleepers.

Indeed it does. That's a terrible idea, by the way. Very unhealthy. Wooden sleepers, utility poles and the like contain creosote for anti-rotting purposes and that stuff is carcinogenic. Definitely not to present code. :)

Yep, we had some old sleepers in an old garden bed that were breaking down heavily and my mother would warn us not to get splinters.