I always thought that duplicating on .net was legal but you were not allowed to sell them. I saw a thread about duplicating sand 2 years ago and thought the Same rules applied for duplicating rails. I would like to check on the forums to make sure. So please reply if it's legal or illegal.
Duping sand/gravel (which isnt possible anymore) is legal as long as you dont trade it. Duplicating anything else is ILLEGAL and will result in being perm banned.
Just to add, because rails are legal to trade, it means that duplicating them poses an issues, especially when powered rails require a substantial amount of gold to make.
As they stated. Sand and gravel duping was legal as long as you didn't trade, back when that was possible. Rail duping is illegal in any form. Even if you plan to use anything you dupe on your own island.