Why don't you take train from Bangkok to Cambodian border?

https://www.railwaygazette.com/news/news/asia/single-view/view/thailand-launches-cambodian-border-service.html

I hope there will be through train service from Bangkok to Phnom Penh.
It will be competitive against long haul bus between Bangkok and Phnom Penh.
But, it won't be competitive against aircraft. So, high speed rail along the conventional rail line will be needed.