ACR TAXIES ONTO ACTIVE RWY WITHOUT CLRNC.

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OUR INITIAL TAXI INSTRUCTIONS WERE TO FOLLOW ANOTHER ACFT FOR AN INTXN TKOF ON RWY 28L AT THE INTXN OF RWYS 23 AND 28L. THE INSTRUCTIONS WERE LATER MODIFIED FOR US TO CONTINUE ON OUR OWN AFTER THE OTHER ACFT DECIDED TO TKOF AT THE F1 INTXN. CLRNC DELIVERY HAD GIVEN US A RELEASE TIME AND WHILE ON TAXIWAY D THE CTLR RECONFIRMED THE TIME; THEN RETRACTED IT AND TOLD US HE COULD GET US AIRBORNE IMMEDIATELY. THEN HE TOLD US TO TAXI TO RWY 28L VIA RWY 23; PROCEED ONTO AND DOWN THE RWY. I SWITCHED TO TWR FREQ AND CONTINUED THE BEFORE TKOF AND TAXI CHKLISTS. AS WE APCHED RWY 28L OUR TAXI SPD WAS DECREASING AND I THOUGHT WE WERE GOING TO STOP. I LOOKED BACK INTO THE COCKPIT TO FINISH THE LAST FEW CHKLIST ITEMS. SHORTLY AFTERWARD TWR CALLED; THE NOSE OF THE ACFT WAS MOVING ONTO THE EDGE OF THE RWY AND TWR WANTED TO KNOW IF WE WERE READY FOR TKOF SINCE WE WERE ON THE RWY. WE WERE CLRED FOR TKOF AND AIRBORNE SHORTLY THEREAFTER. NO ACFT WERE ON FINAL AND WE DID NOT CREATE A CONFLICT. I HAD ASSUMED THE CAPT WOULD NOT GO ONTO THE RWY UNTIL BEING CLRED BY TWR. SLOWING DOWN AS WE APCHED THE RWY FURTHER CONVINCED ME HE WAS GOING TO STOP. HAVING ARRIVED AT THE RWY AND THINKING THE CAPT WAS MANEUVERING TO A STOP I CONTINUED THE CHKLISTS. THIS DIVERTED MY ATTN INSIDE AND I DID NOT REALIZE WE WERE ENCROACHING ON THE RWY. AN UNFAMILIAR ARPT AT NIGHT; A SHORT TAXI; AND NUMEROUS CALLS FROM GND CTL DELAYED THE COMPLETION OF THE CHKLISTS UNTIL REACHING THE RWY. THOSE FACTORS COUPLED WITH THE LONG DAY (10 HRS) COULD HAVE CONTRIBUTED TO THE CAPT'S MISUNDERSTANDING OF THE TAXI INSTRUCTIONS. HE STILL MAINTAINS THAT WE WERE CLRED ONTO THE ACTIVE BY GND CTL. I; ON THE OTHER HAND; HAVE NEVER BEEN CLRED BY GND CTL ONTO AN ACTIVE RWY AND WOULD NEVER ENTER THE ACTIVE WITHOUT TWR APPROVAL. THIS IS PROBABLY WHY I WAS SO CONVINCED THE CAPT WAS SLOWING TO A STOP AS WE APCHED THE ACTIVE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES SEVERAL FACTORS INVOLVED IN THIS SITUATION. GND CTLR HAD A SENSE OF URGENCY IN HIS COM. KEPT REPEATING INSTRUCTION WITH NO CHANGE AND USED 'WE'LL GET YOU OFF RIGHT AWAY' EACH TIME. ONLY 4 ACFT ON FREQ BUT MADE IT SOUND VERY BUSY. THE RAMP AREA IS HUGH AND AT NIGHT YOU JUST POINT IN THE DIRECTION OF TAXIWAY D AND SEARCH. IT IS HARD TO LOCATE. RPTR HAD DELAYED CHKLIST UNTIL SURE THEY WERE ESTABLISHED ON D THEN BEGAN PROCS. LOOKING AT ARPT DIAGRAM HE FEELS IT MAKES NO SENSE TO TAXI ON RWY 23. SHOULD NOT USE IT AT NIGHT.