The plans for the two lines - Bangkok-Hua Hin (Bt90 billion) and Bangkok-Pattaya (Bt100 billion) - would be submitted to the Cabinet for approval.
He expects the signing of memoranda of understanding for the construction around September or October. The construction period will last about three years.
Only Thai companies will be allowed to bid for the projects, he said.
The MoU for three long routes - Bangkok-Chiang Mai, Mae Sot-Phitsanulok-Khon Kaen-Mukdahan, and Kanchanaburi-Bangkok-Map Ta Phut - is expected to be signed in Japan during May 27-28. Construction is expected to start in the second quarter of 2016.
Prajin added that Thailand and China will finalise in September on the financial cost of two train projects with combined length of 873 km. The first will run from Bangkok to Kaeng Khoi and then Map Ta Phut. The second will link Kaeng Khoi and Nong Khai.
He said the construction of the first phase - Bangkok-Kaeng Khoi may start in October.
Aside from rail network extension, the Transport Ministry this year is awarded Bt140 billion in this fiscal year to invest in the road network. Additional Bt45 billion was topped up early this year, Prajin said.