A user from IMDB forum, fireinthewronghole, made a great explanation about Byousoku 5CM ending. An excerpt from the last paragraph that sum it:
So because of the song, we can interpret their meeting at the train tracks as either real or imaginary. It’s possible, like the song implies, that Takaki simply sees Akari wherever he goes. Either way though, the meeting itself is a test of sorts for Takaki and it was meant to completely summarize their whole relationship in one simple but poignant scene. Again, the train tracks and the trains themsevles--being separated by the tracks and the time it takes waiting for the train to pass--is symbolic of the spatial and temporal distance created between the two and how after the trains have passed, Takaki is still left waiting while Akari has moved on. Takaki accepts this, smiles, and walks away as well, showing how he acknowledges that it’s time for him to move on as well. 
This is what I call a Romanticism ending and I think it quite fit with Western audiences. I recall in almost all Hollywood film I've watched, there would be a conclusion for every part. Even if the movie is Post-modern, the audience is swayed into some conclusion. As far I can see, even if the movie trying to go to open interpretation, it still drives into a conclusion.

Well, since this Makoto Shinkai's movie is based on his personal experience, we may have to care that much. Even Shinkai himself said in an interview with NHK that this movie was about telling himself. At that time, he has ended his relationship with his girl.

For Romantics, this movie is bitter sweet. But, I think I want to add some other interpretations. Just some twisted analysis that I would found if it was in other twisted genres. Let's make some interpretations of his smile in the last scene.

Tragedy

I guess his mind was snapped. His imaginary thoughts and the mess of his life made him realize: his life was not meaningful anymore. His struggle with trying to move on made him suffer a lot.

He couldn't catch Shinohara anymore. He was shameful because he was the one that ruined the relationship. He was too scared to face her.

Drawn by his regrets, he couldn't face reality. In his mind, Shinohara is the one that so sweet that he met years ago. He was too afraid to meet the real Shinohara. His love was too strong for this Shinohara of his mind.

Like many world novelists that drawn into their works. As they are fascinated by the story they've made, they also felt envy. This jealousy made them comparing their lives as a main protagonist. Soon, they made their decision about to end this novel in which God has written.

The more he try to move on, the more he found out that the mind won't budge. The story kept stuck into his mind. Toono knew that he couldn't walk straight anymore. He knew that his heart wasn't in his side anymore.

Thus, as the train was passing by, Toono made a decision. This time, as a man who wanted his life in control, he would made a decision about his life. His bitter sweet smile was his resolution from chaotic equation in his mind. He smiled because this time he found control in his life.

His body later was found unrecognized after getting hit by a train.

NB: Based on custom of East Asian which they aren't that reluctant to end their own lives.

NTR

Toono smiled and hired a private detective. He tried to find his long lost love. Like a ghost, he tracked and hounted the young family of his love. Little by little, he was introducing himself to Akari's live.

At first, Akari was refusing to give into temptation. She retaliated to the idea of having an affair. But time and space was not at her side. Like the last love she had, she felt lonely. Her husband was parting away from her. The daily routines of a young worker trying to make a path that his husband was doing made a hole in her life.

Slowly, her longing for touch couldn't keep her away from her long lost love. Like other females, she couldn't forget her first love. Even if she tried to walk anew with her current love. The years she spent on platonic love entangled her.

Thus, they began their relationship and kept stronger as the day goes by. In the end, Toono got his girl back by force. His baby is now inside her womb.

NB: Based on the recently popular tag of Hentai.

Horror/Suspense

Toono smiled and found out that he was getting far too ahead with his mind. He decided to go to a shrink and was having rehabilitation. As the day gone by, he could regain his composure and was making new chapters of his life.

He made new friends in his classes. There was one that enticed him the most. This charismatic person was so fascinating. She was different from the long lost love he had. Her word was so strong that he couldn't believe it that he was able to spent hours to talk with her.

This person charmed Toono as she engaged him with many ideas about life. At first, Toono just wanted to have conversation with her. As the time went by, Toono was more intimate into this female's ideal.

She then revealed Toono that she was a mistress of chaos. She thought Toono as the chosen one. Then, she invited Toono to her exclusive group. In this encampment, Toono made many discoveries about everything. As if he was on a cocoon and that encampment released him from his state to become a beautiful butterfly.

On a late night dance, she made Toono drank a liquid. Toono lose grip and in his head was voices that once suppressed. Among all of the voices, he listened to one voice. That voice was materialized as his own image holding a water staff.

The voice explained that he was an avatar of chaos. He was the real leader of this group. He said that the girl was her mistress. Surprisingly, he told Toono that he was Toono otherself. Toono was a chosen one, selected by an ancient god. Many goals were injected into Toono's mind as he began transformed inside.

Toono now is a new man. His leader gives him new purpose to his life.His God Cthulhu was very precise about his life. He now embrace the chaotic voice from his heart. He has a mission.

Newspaper often wrote about missing people from time to time.

NB: Based on the fact that Japanese have many sects and occultism interests.

Pragmatic

As Tohno smiled, he began to run his life again. Sometimes, the bitter past haunted him. But, like other salarymen who was living in 6-tatami apartment, he couldn't care much. There were bills waiting to be paid.

In the end, Toono just an average guy with bitter past and spending his time from time getting drunk.

NB: Based on the fact that most people can't fully move on and pathetically affecting that one to not being able making breakthrough decision.