That's what I thought, the station is located at the end of the existing 'slow' line.
https://www.google.com/maps/place/สถานีรถไฟอู่ตะเภา/@12.6602151,100.9675533,3222m/data=!3m1!1e3!4m6!3m5!1s0x31028bfddcd3de29:0xf5dd0c02115732e4!8m2!3d12.6615462!4d100.9652668!16s%2Fg%2F11gnpkvnf9?authuser=0&entry=ttu