There are many trains running throughout the day from Bangkok to Surat Thani and return. The Surat Thani train station is about an hour from the ferry pier where you catch the boat to Koh Samui. You will need transportation from the railway station to the pier. The most popular trains are the sleeper trains where you travel at night and arrive in either Bangkok or Surat Thani in the morning.
While the trip is long and the noise of the train may wake you up often, new friendships are sometimes made during the journey. Don't forget to take a bottle of water and tissue paper on board with you. 
The conductor will wake you prior to your early morning arrival in Suratthani, but you might want to take a small alarm clock with you in case.When you disembark at Suratthani, employees of the bus company will be waiting to stow your bags in the correct bus that will take you to the ferry pier for the boat ride to Koh Samui.
When you board the ferry, if you're still sleepy, you can buy a seat in the "VIP" air-con lounge for 40 baht. The seats recline and are very comfortable.
These Rail-Bus-Ferry tickets can be purchased at the train station in Bangkok. For the return trip from Samui, you can buy them on the island at Tropical Easy Travel and though other travel agents.
