Aw, you know how it is. You're like CHOO CHOO and your best friend's all CHOO CHOO and then you're all laughin' at that mess and Mommy's like "What's up wit dat?" and she don't even know. That's trainbros, man. That's what it's about. Bond for life.
Hey, don't let the gender title fool you. Any gender can be trainbros. It's just that writing "trainsis-es" opens up all sorts of language debates and we didn't really want that in the title. But if you say it, it won't look weird, right? So say it all you want. Still cool.