▁train ▁protection ▁system ▁( AT P ), ▁which ▁super vis es ▁both ▁manual ▁and ▁A TC ▁operation ▁and ▁stops ▁the ▁train ▁well ▁before ▁it ▁would ▁enter ▁an ▁occupied ▁block . ▁ ▁At ▁switch es ▁entering ▁into ▁pocket ▁tracks ▁in ▁a ▁direction ▁reverse ▁to ▁the ▁normal ▁operating ▁direction , ▁there ▁is ▁a ▁" P ▁Sign al ". ▁The ▁P ▁Sign al ▁is ▁a ▁single ▁lun ar ▁signal ▁that ▁displays ▁the ▁letter ▁" P " ▁and ▁is ▁found ▁at ▁the ▁entrance ▁to ▁some ▁but ▁not ▁all ▁pocket ▁tracks ▁and ▁other ▁selected ▁main ▁line ▁inter lock ings ▁in ▁the ▁reverse ▁direction ▁of ▁the ▁normal ▁flow ▁of ▁traffic . ▁Below ▁the ▁P ▁signal ▁on ▁the ▁same ▁post ▁is ▁a ▁p unch ▁box ▁to ▁allow ▁the ▁train ▁operator ▁to ▁manually ▁set ▁the ▁switch ▁to ▁the ▁diver ting ▁direction ▁( reverse ) ▁when ▁the ▁P ▁Sign al ▁is ▁lit . ▁The ▁P ▁signal ▁is ▁only ▁lit ▁when ▁a ▁train ▁occup ies ▁the ▁two ▁track ▁circuit ▁approaching ▁the ▁P ▁Sign al ▁and ▁all ▁possible ▁routes ▁into ▁the ▁inter lock ing ▁are ▁not ▁occupied . ▁ ▁Similar ▁signals ▁are ▁also ▁in ▁place ▁on ▁the ▁Baltimore ▁Metro ▁Sub way Link , ▁the ▁P AT CO ▁Speed line , ▁and ▁the ▁Miami ▁Met ror ail . ▁ ▁Sign age ▁ ▁Two ▁track side ▁signs ▁exist : ▁The ▁" S " - Sign ▁is ▁posted ▁at ▁re venue ▁tracks ▁and ▁indicates ▁to ▁the ▁operator ▁that ▁a ▁station ▁is ▁ahead . ▁It ▁is ▁located ▁at ▁a ▁distance ▁no ▁greater ▁than ▁ 1 2 0 0