The Baht 85.3bn ($US 2.6bn) project includes the construction of 26 stations, 26 overpasses, 102 underpasses and four tunnels.
The project will be tendered in three sections ranging in length from 84km to 135km.
The line will terminate near the Chiang Khong-Huay Xai Friendship Bridge, which carries road traffic between Thailand and Laos across the Mekong.
Construction is expected to begin next year with commissioning scheduled for 2022-2023.
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