If they are creosoted, it doesn't penetrate through, so the outside edges could be cut off, still got to get rid of that though (and it's pretty carcinogenic for humans too!).Creosote won't do your plants much good.
I know you can't buy creoSote any more (only creoCote) but I assume you can still buy old treated sleepers, I'm pretty sure they had some in a local yard to me when I was looking at wood for garden edging a year or so agoI thought it was now illegal to sell creosoted sleepers but don't recall where I read it. Also restrictions on where they can be installed. Might be worth some research before spending any £££.