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Since the plan is to go to Hat Yai via the Sungai Golok railway line, the easiest way to access it would be by express bus from Kuala Lumpur.
Since November 2014, all express buses to eastern states have been moved to the Terminal Bersepadu Selatan (TBS) or Southern Integrated Terminal. With the recent move of express buses to northern destinations to TBS in November 2015, TBS now serves express buses of all destinations in Peninsular Malaysia, not just for southbound buses only.