When Doc is out on the train trying to get the train to blow the last log hence letting them hit 88 MPH. They begin talking on their walkie-talkies. Doc slips off the train rail and Marty says into the walkie-talkie, "Doc, Doc I'm going to slip you the hover-board." Doc is holding on the the rail with both hands and there is no way that he can hear what Marty said. But he acknowledges with a head nod.