My GPS say it is 700 Km, and 8 1/2 hours currently, non-stop using roads.

A train route is probably a bit longer; and trains often stop to drop-off and pick-up passengers. The Special Express trains make ~ 18 stops.

"High-speed" rail here means max. 200 KpH; with Bangkok-Chaing Mai projected at 6-8 hours (the "government" likes to quote 5 hours). Any high-speed rail project is at least 20 years out.

Obviously an airplane is able to fly direct, non-stop (one hopes) in an hour or so.