Padang Besar is the only direct rail link between Malaysia and Thailand. It also has a road crossing between the two countries which is a lot less busy than the more direct Sadao-Bukit Kayu Hitam crossing.
The town on the Malaysian side of the border, in Perlis, is also called Padang Besar. Malaysians usually refer to the town on the Thai side as "Pekan Siam" or "Siamese Town".
Slightly bigger and a lot more ramshackle and sleazy looking than its Malaysian counterpart, Thailand's Padang Besar is often busy with Malaysians crossing the border for shopping and to indulge in the freewheeling entertainment which Thai border towns are infamous for.
Both [http://www.ktmb.com.my Keretapi Tanah Melayu]'s '''Senandung Langkawi''' express between Kuala Lumpur and Hat Yai and the [http://www.railway.co.th/english/ State Railways of Thailand]'s '''International Express''' between Bangkok and Butterworth in Penang stop for a short while before proceeding to or after coming from Malaysia. The Senandung Langkawi departs for Hat Yai at 08:45 and for Kuala Lumpur at 15:45. The International Express leaves Padang Besar for Bangkok at 16:30 and for Butterworth at 07:20.
Note that Malaysia is one hour ahead of Thailand. When it is noon in Thailand, it is 13:00 in Malaysia.
The border crossing is open 05:00-23:00 Thai time (06:00-midnight Malaysian time).
'''Bus to/from Hat Yai:''' Regular local buses link Padang Besar and Hat Yai via Sadao. Journey time is about one hour. Buses leave from the main town square. The bus can also be caught on the main road right outside the Thai immigration complex. Wait on the opposite side of the road for Hat Yai-bound buses.
'''Bus to/from Malaysia:''' There are no direct buses. From Hat Yai, get off the bus at the Thai immigration complex and walk or take a motorcycle taxi (20 baht or RM2 should be enough). If you choose to walk, remember that the two immigration checkpoints are several hundred metres apart. There is a duty free shopping complex in between the checkpoints on Malaysia territory.