I read this as bullying resulting in self-harm or something like that, with Koma mentioning Yura did things to herself (not standing up against bullies, harming herself, etc). Koma felt like a scumbag to me from start to end. She truly is a coward but also someone who is pretty self-serving. The train felt to me like Koma is stuck just going along with things. She's just being driven by others, a passenger in her own life because she's unable to figure out where her own station is (either because she can't decide or just doesn't know it). She's pretty aimless, doesn't really have anything going for her (average grades, no partner, no club, etc) and the only unique one she has is a childhood friend dependent on her so she loves that feeling. In the end, she doesn't even have that so her world really is ending.
Yura, on the other hand, felt like she kept giving Koma chances until she has that ultimatum. I agree, she's strong and I'm glad she finally decided to step away from Koma in the end. Yura's world also ended that day but it's likely one that's freeing compared to Koma's stasis.