Its a short monorail proposed to link across to the extended green line on the eastern side of the airport
https://www.bangkokpost.com/thailand...mueang-airport
Airports of Thailand Plc plans to build a 3-kilometre monorail electric train route to link Don Mueang international airport with the Bangkok Transit System (BTS) Green Line on the Mor Chit-Saphan Mai-Khu Khot route in Bangkok's northern suburb, Don Mueang airport deputy director Flying Officer Sumpun Khutranon said.