The plan covers the 132-kilometre stretch of track between Saraburi's Map Kabao and Nakhon Ratchasima's Thanon Jira junction, the 165km route linking Nakhon Pathom to Hua Hin, and the 118km of track between Lop Buri and Pak Nampho in Nakhon Sawan.
It will also cover the 167km Prachuap Khiri Khan-Chumphon route and the 185km Nakhon Ratchasima-Khon Kaen route.
The plan, however, will remain on hold until it can be examined by the new government, Mr Prapas said.
The five dual-track routes are not included in the Pheu Thai government's 2-trillion-baht infrastructure development plan, he said.
The fate of that scheme remains uncertain, as the Constitution Court last month accepted for consideration a petition challenging its legality.
The SRT plan has already been approved by the cabinet, which agreed to set aside 176 billion baht for rail development, Mr Prapas said.
Construction is expected to commence once environmental impact assessments are completed, he added.