, ▁and ▁" I " ▁and ▁" O " ▁are ▁not ▁allowed ▁to ▁be ▁used ▁as ▁route ▁letters . ▁The ▁M ▁Route ▁would ▁likely ▁be ▁assigned ▁to ▁a ▁future ▁route ▁to ▁Lin col nia , ▁branch ing ▁off ▁from ▁the ▁C ▁Route ▁south ▁of ▁Pent agon ▁station . ▁ ▁Sign aling ▁▁ ▁F ixed ▁signals ▁are ▁only ▁present ▁at ▁inter lock ings , ▁and ▁consist ▁of ▁three ▁l amps : ▁two ▁red ▁and ▁one ▁lun ar . ▁They ▁show ▁a ▁red ▁over ▁red ▁aspect ▁for ▁" stop ▁and ▁stay ," ▁a ▁lun ar ▁( white ) ▁aspect ▁for ▁" pro ceed ," ▁and ▁a ▁flash ing ▁lun ar ▁aspect ▁for ▁" pro ceed ▁on ▁diver ging ▁route " ▁( only ▁present ▁on ▁newer ▁inter lock ings ; ▁older ▁inter lock ings ▁are ▁being ▁updated ▁with ▁signals ▁that ▁show ▁this ▁aspect ). ▁Tra cks ▁are ▁divided ▁into ▁block ▁sections , ▁which ▁can ▁only ▁be ▁occupied ▁by ▁one ▁train ▁at ▁a ▁time , ▁a ▁method ▁to ▁protect ▁against ▁coll isions ▁used ▁by ▁most ▁railway ▁systems ▁world wide , ▁but ▁there ▁are ▁generally ▁no ▁fixed ▁signals ▁to ▁protect ▁block ▁sections ▁( sign als ▁tend ▁to ▁be ▁only ▁at ▁beginning / end ▁of ▁route ▁and ▁just ▁before ▁track ▁switch es ). ▁Virtual ▁signal ▁aspects ▁are ▁transm itted ▁to ▁the ▁train ▁by ▁a ▁cab ▁signal ing ▁system ▁and ▁displayed ▁to ▁the ▁operator ▁in ▁the ▁cab , ▁as ▁well ▁as ▁transm itted ▁to ▁the ▁A TC ▁system . ▁Entry ▁into ▁an ▁occupied ▁block ▁is ▁prevent ed ▁by ▁an ▁automatic