Fence rails usually have a lot of sapwood in them and its treated alot of time. I've got quite a bit of poplar fence boards around one of the paddocks actually.
If you can get the heartwood it has a greenish cast to it with a really tight clean grain. Some love it some don't. I've used it for some clocks and furniture and that sort of thing, its fairly nice for carving and I like its looks.
Its soft though, you'll want a pretty durable finish, something that won't flake off if the wood dents a little bit.