From the linked article:
As it stands, Hua Hin Airport already deals with approximately 30,000 private jets annually, signalling its strong potential for growth,..........
If you believe that an average of 82 private jets fly into HH airport each day you probably believe in the tooth fairy. It is an active commercial ab initio flight training school operated by the AOT and in frequent use by the RTP for para drops but, even adding all those landings together would not reach that figure.
I shudder to think of the disruption that would ensue it widening the twin dual-lane road tunnels that pass directly under the runway. No reason is given for the perceived need.
Widening of the rail tunnel could be justified by the current upgrade of the rail system to twin track from the existing single track.