You'd think that there'd be a robust secondary market for treated rot resistant timber that's "too rotten for trains, but perfect for landscaping". Sure, there's chemicals in it but if it's safe enough to be embedded in untold miles of rail bed then surely it's safe enough for comparable landscaping use.

Like everything else there's probably some 2nd/3rd order consequence of regulatory perversion that prevents it. Like because of the circumstances it wrongly gets classified as hazmat or something.