Both projects were put forward by a public-private committee.
Track doubling of the Bangkok - Nong Khai line will be completed in 2019, six years earlier than originally planned.
The 336km new line will run east via Maha and Roi Et to Mukdahan, and then head north paralleling the Laotian border to Nakhon Phanom. The project is expected to be completed in 2017 at an estimated cost of Baht 41.4bn ($US 1.3bn). The new line is forecast to carry 5.4 million passengers and 250,000 tonnes of freight in its first year of operation, rising to 133 million passengers and at least 6.3 million tonnes by 2037.