It starts at 7:47, when Tai and Yamato (Matt) are arguing inside the Ferris wheel. It's a really sad but poignant song, and I believe that they may have even played this song in season 1 of Digimon. I would really want a link to this soundtrack.
Never mind, I just found out from Reddit that there is no officially released list of soundtracks for Digimon Tri, except for the singles. I hope the OST comes out soon.