It's a term used in Miami Florida It's meaning stems from the word "Train", performing group sex on one female who is willing.
Usually men find a girl who is fucked up enough or down to fuck. Some guys fuck a girl in the dark then
call other guys to come through and fuck their girl when they are bored and thats how some trains start.
Jimmy is fucking lisa in his room, "Lisa, I'm getting tired, lets take a break", "No Jimmy, I haven't gotten off I need to have at least 10 orgasms before we can stop". Jimmy looks at Lisa sideways, while still fucking her he picks up his phone in one motion. "I am going to call my friend Luis so he can help me out" Jimmy calls Luis, "Yo luis, my girl Lisa wants 10 orgasms and I can't fucking do that, you wanna call a couple homies to come trainbomb this bitch"...."Jimmy, I'm so excited..."
A person you hate intensely not because they deserve such hatred but because they serve as a focus for your general hatred. A healthy way to dispense with excess animosity.
Jake's not really that big an asshole. He just serves as my hate sink.