[TOMT] series that involves gang violence

I vaguely remember there being a teenage girl (main character) who’s mother thinks she’s trouble and she wants to send her to boarding school but her grandpa stands up for her (he lives alone in an apartment) so she ends up living with him. I think he really dislikes her boyfriend because he’s involved in that gang lifestyle. I know that the granddaughter/her boyfriend’s spot to chill is on top of a train car. The grandfather is pretty ruthless and he’s friends seem to be pretty cut throat too (in the badass protect those you love kind of way).