A 6+ person slap, where 1 person holds the slap-ee and 5+ others run at full speed behind each other, slapping the slap-ee one after another, acting like train carriages behind the first person. This is done most effectively when the slap-ee's head is held firm so they cannot shy away from the on-coming hands. A Train Slap is usually called upon when somebody has done something wrong and deserves severe punishment.
Guy 1: Dude?! Did you hear that Matt got an 8 person Train Slap for hitting on Marc's girlfriend?
Guy 2: Awwwww dude! That woulda' hurt like sheeeeeeet.