High Speed Rails extended from Bangkok to inland cities of Thai wouldn't pay back building cost. But, they must be meaningful, if they are connected with neighboring nations, and form high speed rail networks in Soth East Asian counutries.
So, as well as the high speed rail between three airports, the other 2 high speed rails should be built.
The rail to Chiang Mai should be extended to China via Myanmar, while the other should be extended to Vietnam via Laos or Cambodia.