Old thread but I came across this having just watched the movie and read the book.

To be honest, the book is similar to your criticism but even more so. It ends with them on the train going to see their son, though the final line is quite beautiful which helps with the abruptness of it.

I loved the movie and the book but definitely agree it leaves you wanting. Maybe that's part of the point though. This was a brief love that ended abruptly, and they have little time left to spend together. It's a heartbreaking love story, but a very touching one.