I am wondering how far in advance (if at all) I would have to buy a ticket from Bangkok to Krabi (via Surat thani) in early Feb. Is it that busy at this time of year or can I just show up at the train station that day to buy my ticket?
Also, does anyone know how much the the fare is for the overnight train?

I have no experience with the train trip but before you committ to a long gruelling rail trip, have a look on the air asia web site. We booked fights about 4 weeks in advance and paid less than $40 aus inc taxes last November.

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I love Thailand this much

We caught the train only as far as Hua Hin, but that was enough! The seats are quite narrow and not a lot of places to put luggage. The train was quite crowded - lots of people standing. I'd be inclined to fly as I think you'd be a bit stiff and sore by the time you went all that way by train.

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You will need to take the train to Trang, and then get a mini bus from the travel shop opposite the railway station to Krabi (about 3 hours I think). If you decide the train over flying I would get the 2nd class sleeper which is very comfortable. It may be good to check at the train station the day before travel just incase it is full.

A really good web site and travel consultant is mattana@travelconnecxions.com They can make the reservations for you in advance, have the tickets delivered for a small fee to you hotel and give you all the information that you may need about the trains, including schedules. If I were to travel by train I would definitely go at night and get a sleeper. You save on hotel rates and at the same time get to your destination.
However, for a few $$ more, and to save lots of time, I would just fly and get it over with. Unless I were travelling to Chiang Mai where you can get some great scenery into the trip near Chiang Mai, I wouldn't bother with the train.
PEACE
David