Ayutthaya railway station is not on the island , you can go to the island by a ferry just a very short way, pier is on the opposite side of the station. Walk across the main road and go down the small road, a boat ran every 5 mins for 4 baht or you can take a bus at the bus stop at the front of the station. Be sure to check with driver for direction before get on, 7 baht.