Hi, its been driving me crazy I've been looking for this one shoujo manga everywhere but i just cannot remember the name, I remember some of it ...
Main girl moves from a small town to the city, she becomes the interest of popular guy.
The guy confesses to her and she only says yes because she wanted friends.
Sometime later their at school near the lockers and the guy wants to get it on..she becomes shy and doesn't want to, so he tries to force her until another guy intervenes.
She becomes closer to this other guy, who loves trains and takes her to his house (lets just call him train boy) and wants to break up with her boyfriend
Her boyfriend sees them becoming close and starts becoming psycho.
her boyfriend gives her a hair clip to "mark his territory".
train boy and her kiss, and her boyfriend sees.
sometime later after school train boy, main girl , and her boyfriend are on the school roof..train boy and her boyfriend fight, they get close to the edge she tries to stop them her boyfriend tries to pull her off the roof of school but train boy stops him and her boyfriend end up grabbing the clip and falling to his death.
Now they have to lie and cover up his death..

Sorry for the long vague description.

Sounds really familiar... is it Sekai no Hate?