The movie is one of the saddest, most wretched, most beautiful things ever made, with a touch of Heartwarming Moment at the very end, where Tohno might've had the chance to find Akari again. But she's engaged to someone else by then.
The last scene depicts young Takaki and Akari at the same train track from the start of the movie, where Akari goes across the track but the train comes, stopping Takaki from getting across. At the start when the train gets by, she's still there waiting, while in the end Older Takaki thinks he sees her, but when the train goes by she's not there anymore.
As most people will point out, the whole film will reduce you to tears, but one of the most painful to watch Anguished Declaration of Love ever put to film happens about halfway through:
Kanae: I guess my wish is to be with Tohno will never come true...but I still...But I still won't be able to stop loving him. Not today, not tomorrow, not ever. I fell asleep that night crying...the only thoughts in my head being of Tohno.
About halfway through the first part, as Takaki is reaching into his coat pocket to get money to buy from a vending machine, a letter he had written for Akari and intended to give to her in person falls out and is blown out of sight by a strong wind. The look on his face following this is heartwrenching.
The manga shows just how deeply Takaki continues to hurt from his unresolved feelings for Akari, all the way into adulthood, that his attempt to confront it by going back to Iwafune with Risa leads to him having a BSOD bad enough that he fails to get off the train, as well as how it hurts others around him like Risa.
The last few pages of the manga's Chapter 10 show an hallucinatory young Akari bidding farewell to Takaki's retreating form, emphasising once more that, even if he's finally managed to let go and move on, he's lost so much.
One more side shows in painful detail just how hard Akari took her family being relocated to Tochigi.
After One more side covers the events at the railway crossing, it delivers one final Gut Punch: The contents of the letters Akari and Takaki wrote to each other but never managed to give, which reveal beyond doubt that, at least at the point of writing, they loved each other.