The ending scene can be taken both ways, maybe she was there, or maybe not. For me, I believe she really was there. This may be subtle, but as they passed by each other, both of them (even the girl) seemed to notice each other. However, when they look back, as the gods of love like to interfere, a train appeared.
As the train is passing by, Akari went ahead. I think it symbolizes that she had already moved on for a long time. However, the main character clearly still had lingering feelings for her, as seen in episode 2 and 3. He did not only look back, but he also waited for the train to pass. He even turned around and took his hands out of his pocket. However, when the train passed by, the woman was nowhere to be found. He took it as a sign to finally moved on, and as he was smiling, he turned away. If I remember correctly, he was sad in episode 2, and even more depressed in episode 3: he even tried to slept with other woman, and he even quit his job to forget the girl.
We don't know for sure who first stopped writing letter, but I think it was the boy. This can be seen in the scenes where they exchanged letters, though I don't think it means that the boy stopped liking her — as seen in the ending, he truly loves her. Maybe it was due to some other reason, like change of address, undelivered letter, or something we would never know. But we can see that even though they don't receive letters from each other, they still looked at the mailbox - both of them. Even when the girl was walking with another guy, she still looked at the mailbox.
Anyways, no matter how anyone views it. The movie was a masterpiece.
By the way, I remember reading an interview with the writer/director of the movie where he said that the theme for 5cm/sec was "reality" - it isn't always a happy ending in real life, most of the time love fails. (Sorry, but I don't have a source for this).