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.

When you eat dirt for the first time and leave your family to build a shack in the woods

To be "riding the train" means to be single and in pursuit of someone to spend the day and/or night, without the intent of perusing a relationship with that person.

On Valentine's Day the only way to "get off the train" would be to find a date, to give up in the pursuit of finding a date or when the clock strikes midnight and you're all alone.

Related: To be on the train / To be on the desperation train / To be on the train of despair.

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