The speaker remembers the final time they saw someone they love. That person said "I love you," and then left on a train. At the time, it was just a goodbye, but looking back, it became the last moment they would ever share.
The speaker calls it "the best day but also the worst day" of their life, and that contradiction is the heart of the poem. It was the best because they heard those words. It was the worst because they now know what came next, even though they couldn't have known it while the train pulled away. The poem doesn't say what happened to the other person, and it doesn't need to. The loss is what matters.
The mood is quiet, raw, and sad. The plain, simple language makes it feel like someone speaking honestly through grief, with regret for a moment they didn't know to hold onto.