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similar to the oral triangle, when a minimum of three people are performing oral sex on each other, and the people are in a line, forming a 'train' by their respective bodies being 'linked' by the oral-genital contact.
Suzie was unable to answer her phone because she was too busy eating out her girlfriend while her boyfriend was behind her, eating herself out at the same time.

Instead, she texted to her best friend, "can't talk, in the middle car of an oral train"
by Cysteine August 09, 2011