I can confirm that having been to The Gambia three times there is no railway network. The best way around is either local bus or bush taxis (not green not yellow ones) the ones that take locals around. Me and my family did a 4 mile trip between Senegambia and Kotu for just 15D (30p).
If you there a week/two weeks or however long, get in with a guide and make sure you hussle them! You'll get taxis cheaper. They can seriously rip you off if you don't the system, we did for £100 (£20 to the guide) a trip to Juffereh and St James Island (ferry included) and Paradise Island in one day. Half of what it should have been.
If you are in Gambia on a Sunday pay a visit to Mamas pig roast at Fajara, well worth the visit! An absolute feast, all you can eat.