I also have the same question, since there's some contradicting info online. I suggest to check with your hotel in Hua Hin before hopping on the train. As for the taxi the estimated taxi fare from Bangkok to Hua Hin is around 2,500 to 3,000 Thai Baht, depending on the negotiation with the driver and the type of taxi you take. Make sure to settle the fare beforehand to avoid any surprises at the end of the trip.