A gang with 4 or more members
Example

G-train
K-train
And the legendary F-train
What’s up guys train gang here today I find the legendary cure for cancer
by The Bunnings sausages June 1, 2018
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When a single individual is, "made love to", by several groups of individuals, placed in "cars", all at once and in corresponding fashion. To be considered a true "gang train" each "car" must consist of two people, typically male, while each "train", is made up of at least three "cars". Similar to "running a train" or "gang banging" someone.
Dane: Yo fam! That slut Sheila said she wanted to smash, but only if we could get a gang train going.

Alex: Well, I'm down; so we have at least one car. Now we need a minimum of four other people to successively take turns having sex with her in groups of two in order for us to have a successful gang train.

Dane: Sweet! Let's hit up Ryan McNeal, he'd be down; he'll tap anything. Now to find at least three other people...
by Very Public Alcoholic 1 December 5, 2016
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