registering the clock time of the day when {HEAD ~ said change} of {TAIL ~ state} occurs at each station; and 
registering the clock time of the day when { HEAD ~ said change } of { TAIL ~ state } occurs at each station ; and
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH