It is possible to enter Thailand by air, land, or sea from countries throughout the region and across the globe. Flying is the most convenient mode of transportation for traveling to Thailand, as visitors can fly to Thailand on non-stop routes from many corners of the globe on both international and Thai airlines. Furthermore, Thailand’s central location makes Thailand an ideal hub for exploring the rest of Asia. In addition to the primary international airport located in Bangkok, visitors from abroad can fly to Thailand on international flights destined for Chiang Mai, Phuket, Koh Samui, Krabi, and even Udon Thani and Hat Yai.
It is possible to travel by bus from most of Thailand’s neighbors. From Malaysia, one can cross north by bus to Thailand at various border checkpoints in Songkhla, Yala, and Narathiwat provinces, although most busses will head to the hub of bus travel into, out of, and around southern Thailand, Hat Yai. Busses from Laos originate in Vientiane and cross the Mekong River to the Thai border town of Nong Khai, where they continue for a two hour ride to Udon Thani, from which visitors can arrange other busses or air travel to other parts of Thailand.
Rail lines laid throughout Thailand create a 4,000 km system that is both efficient and comfortable. Passengers can travel by train from Chiang Mai to Bangkok to the Laos or Malaysian borders and many places in between. While the journey on a Thai train generally takes longer and can be more expensive than a voyage by bus, trains are safer and are generally more comfortable. Popular train routes include Bangkok to Chiang Mai and Bangkok to Surat Thani, the launching point for boats to Koh Samui.
With your own vehicle it is possible to travel from the country where the vehicle is registered into Thailand, assuming you have paperwork to prove vehicle ownership and insurance and you drive to Thailand at an approved border crossing.It is not generally allowed for tourists to drive rental vehicles from a neighboring country to Thailand.
Source: http://www.tourismthailand.org/Travel-and-Transport/transportation-options
