Love Flakes are Snowing(2006)

Hitohira no Koi ga Furu

There's a jinx at Haruru's school - if a couple sees the bullet train at 6 am together, they'll be together forever! But if a couple sees the bullet train at 10 pm together, they'll break up!
Haruru is going out with Nagi, a boy who goes to another school, and their love is just starting to blossom. But what happens when Haruru happens to see the 6 am bullet train with someone else!?