When a group of men, ten or more, line up naked, on their hands and knees, as a train. The first male is the engine. The last male is the caboose. The men in the middle are the cars. The cars connect to each other by shoving a one or more fingers up the ass and fondling the prostate of the male in front of each man in the train. The Prostate Express then can move about the room as desired.
Howard held an awesome all male sex party. He invited fifteen guys over and they all hooked up in a Prostate Express and tracked around the house in circles saying "I think I can. I think I can" until they all blew their loads.