{HEAD ~ The control center} knows at any time by {TAIL ~ radio contact} where the train is traveling 
knows by
{ HEAD ~ The control center } knows at any time by { TAIL ~ radio contact } where the train is traveling
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH