Yep, as SeratoninSid and others have pointed out, do not use recycled railway sleepers. They are illegal to use in thiese circumstances, but you will still see them for sale from time to time.
If you prefer a more rustic/abused look to the sleepers you can safely use, then attack the edges with a hatchet/hammer/chains to take notches out, add dents and general signs of wear and tear. This is called 'distressing', and is often used to make furniture look aged (often while disgusing repairs to antique pieces). There are a number of guides to distressing wood online.
Leo