It's a drawing of a choo-choo train. If you gave a five year old boy a tattoo kit and said, "hey li'l fella, whaddya want inked?" chances are he'll say a hairyplane or a choo-choo train.
A man? He'd have guns, bombs, knives and hookers inked on him. Personally, I have an array of Mexican phrases inked, but that's all we had access to in that particular jail.
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I also am subcribe to postal pornography - CAOH