It's not unusual to omit 'on'. It's not wrong.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
AnintitaRail and bus services inRail and bus services in Southern ThailandtheSouthern Thailand
tamguatlayRail and bus services in the Southern Thailand were halted on Wednesday due to heavy flooding.I think in British English, it is necessary to include 'on'.It is but not "the" before "Southern".