This is true for most train tickets. Not for boat tickets though: you can get very cheap return tickets (e.g., for the ferry Portsmouth to Saint Malo v.v., but

This is true for most train tickets. Not for boat tickets though: you can get very cheap return tickets (e.g., for the ferry Portsmouth to Saint Malo v.v., but doubtless many others) that are cheaper than a single fare, and the terms and conditions include a penalty that they say they'll deduct from your credit card afterwards if you don't actually take the return leg of the trip.