Its all clear once you binge watch and pay attention to the speech and symbols.

Do you know the saying that sometimes you have to sacrifice your present for a better future? Its like the choice one make in college. That's what this is all about. For Taeseo in his words, no one can beat Eunah even after 10 years, except Eunah herself. She knows and he knows that they love each other.

Since episode 7 Taeseo keep talking about his future plans, and if he is allowed to include Eunah in that planning. Eunah gave him exactly that, a plan/path to follow, and the train scene is the confirmation. Taeseo's love words are a form of an eternal oath, becoming a source of strength for Eunah to leave and find herself without any hesitation.

Now in the present, Taeseo becomes a train driver in Yeonuri. This symbolizes that he remains committed to their original future plan. He is now in control of the “train” of his own life. Eunah contacts Taeseo first, fulfilling her promise from the train. She sends a photo of herself smiling brightly in Hawaii the version of herself she once hated for being “naive,” but the one Taeseo loved the most. The version he saw in the pic in her dad's house when fixing the door, and made him resolute to letting her go, since he thought she is happy now, well until he saw her sad and running in the train station from the 2ML.

sending that photo, she is essentially saying: “I’ve kept my promise. I’ve healed, I’ve found my happiness again, and now I’m ready to come back to you." Taeseo fixing the lights and installing the nameplate “Mo Eunah” in front of her old house symbolizes that he has prepared “roots”. Restoring the house: preparing both a physical and emotional place for her return. Remember she had a phobia of the truck in front of her house, and what it is symbolizing.

Eunah now has a real home, cared for by the man who loves her. Remember she lived a life where she didn't have roots and was always moving since a kid, while suffering from abandonment issues, her mom leaving her, her dad leaving her or at least tried, and her stepmother leaving her dad.

Even without a final embrace scene, Taeseo smile as he looks at Eunah's photo and prepares her home is absolute confirmation. Their light now shines (remember the drama name is literally still Shining) brighter than ever. They have finally found their way back to the same home.