The Thai Southern Line can take you all the way down the Malay Peninsular, even to Singapore if you change trains at Pedang Besar.
The Malay Peninsular is formed by the Tennaserim Hill system rising from the Indian and Pacific Oceans due to the seam in the earth’s techtonic plates – particularly the Indian plate running into the massive Eurasian plate. This collision also gives rise to the Himalayas, of which the Tennaserim range is the most southerly tendril.
Over the years, this northern part of the Pensisular and its isthmus has been a shifting front between the predominantly Buddhist kindgoms of the floodplains to the north and the predominantly Muslim trading sultanates to the south, with pockets of different peoples in the hills, such as the Mon and Karen. And it is still the case today.