I took the direct regular bus from Khao Sok National Park to Khao Lak in January - very straight forward, there is a bus once an hour which stops opposite the entrance road to Kaho Sok National Park. I stayed in a guesthouse along the road to the park and they ran me up to the bus stop in time for the bus. There is a cafe in case you miss the bus (as happened to me) and have to wait for the next one. Cost was really cheap (I can't remember exactly how much, but well below THB 100) and we stopped for about 10 minutes in a town on the way. In all, it took around 1.5 hours. Unfortunately I can't help you with the Khao Lak / Railay leg.
If you want door to door, luggage and all, and are prepared to pay for a private taxi transfer contact Boss of Cheaper than Hotel Taxis in Khao Lak on firstname.lastname@example.org for a quote including one from Khao Lak to Ao Nang in Krabi where you need to get a boat transfer to Railay Beach. Plenty of recommendations about Boss in the FAQ on the right on TAXI Companis and this in particular that gives his latest rates tripadvisor.co.uk/ShowTopic-g297914-i10908-k…
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