If you want to travel from Kuala Lumpur to the Malaysia-Thailand border at Padang Besar you can take KTM train from KL to Padang Besar, take a bus via Kangar or fly to nearby airports.
Travelling overland, the quickest way to get to Padang Besar from Kuala Lumpur is to take an Electric Train Service (ETS) from KL Sentral Railway Station or the old Kuala Lumpur Railway Station.
A train to the Thai border from KL takes around six hours.
Once you arrive at Padang Besar you can:
There are no direct bus services from Kuala Lumpur to Padang Besar.
What you will have to do is take a bus to the nearest large bus station at Kangar (Terminal Bukit Lagi).
From Kangar Bus Station there are regular MyBas Perlis bus services to the border at Padang Besar taking around an hour.
The bus drops you off very close to the entrance to the border crossing.
From Padang Besar in Thailand, you can then continue your journey to Hat Yai where there are connections to all destinations in the south of Thailand, all the way up to Bangkok.
Another option to get to Hat Yai from KL by bus is to avoid going via Padang Besar and take a direct bus from KL to Hat Yai (they cross over the border at Danok / Sadao).
As most people crossing over the border at Padang Besar will be heading to nearby Hat Yai, you maybe want to think about taking a direct flight from KUL to Hat Yai (HDY).
For cheaper flights, you could take an internal flight from KL to Alor Setar Airport (AOR) then take a cheap Komuter train to the border from Alor Setar.