when your dog or cat, after a particularly difficult bowel movement, decides to sit on the floor, and pull it self forward using its front feet only, dragging it's asshole on the (usually) carpet to scratch the ham hocks (hemmies) off, then gets up, turns around, and sniffs the skid mark
"OMG, look, Coby's had the hershey squirts, came right inside and started riding the train."
Two men, One woman, Woman in the middle of the two men interconnected by penile insertion, one male into the vagina or rectal cavity, one male in the mouth of the woman. Likeness the links of a train.
"She was riding the train last night."
"We pulled a train last night"