It's when someone runs into someone from behind
When two people are tounching when one is behind the other, and are both facing the same way.
"ohh look theres a man boning a sheep lets BONE TRAIN it!"
"ohh look theres Griffo lets BONE TRAIN him!"
when more than 3 guys make an anal sex train. grinding in and out of each other's anus like a locomotive. the guy in front is called the conductor.. all partneers thereafter are called cabooses.
"dude, i was at that gay club the menagerie the other night and these 5 guys were having a wicked crazy bone train."