ACR IN POS AND HOLD HEARS TKOF CLRNC. BEGINS TKOF ROLL. ABORTS AS SEE ACFT HALF WAY DOWN RWY ON TKOF ROLL.

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EARLY MORNING LAUNCH. TWR WAS USING 26L FOR LNDG; 8R FOR NOISE ABATEMENT TKOFS. AFTER WATCHING SEVERAL ACFT LAND AND CLR 26L; OUR FLT WAS CLRED INTO 'POS AND HOLD 8R.' WE DID SO; COMPLETING 6 FINAL ITEMS OF TKOF CHKLIST AS PER COMPANY PROC. AS WE DID THIS; WE OBSERVED AN ACFT (LIGHT TWIN CARGO TYPE?) TAXI OUT A FEW THOUSAND FT DOWN 8R AND HEARD TWR CLR THAT ACFT FOR TKOF WHILE WE WERE TAKING POS AT THE END. NOTE: TWR NEVER TOLD US THEY PLANNED TO LAUNCH THAT ACFT WITH US IN POS. WE (COPLT AND MYSELF) WATCHED THE TKOF AIRPLANE ROLL; BREAK GND; AND MAKE A VERY IMPRESSIVE (TIGHT R TURNOUT); TKOF AND TURN DOWNWIND. TWR THEN MADE A XMISSION THAT BOTH MY COPLT AND MYSELF ASSUMED WAS A TKOF CLRNC FOR US. MY COPLT ACKNOWLEDGED BY SAYING SOMETHING TO THE EFFECT '(CALL SIGN); CLRED FOR TKOF; BUT WE CAN'T DUPLICATE THAT' AND THOUGHT WE HEARD A 'ROGER.' WE TURNED ON LNDG LIGHTS; I GAVE CTL TO COPLT AND WE BEGAN OUR TKOF ROLL. A FEW SECONDS LATER WE BOTH SAW A SMALL SINGLE ENG ACFT TAXIING OUT AND BEGIN A TKOF IN FRONT OF US ON 8R. THIS ACFT WAS ABOUT HALF WAY DOWN THE RWY. WE WERE NOT GOING VERY FAST AND EASILY BRAKED TO A STOP. I CALLED THAT WE WERE ABORTING FOR A SMALL ACFT IN FRONT OF US; ASSUMING THAT ACFT WAS IN ERROR. TWR XMITTED TO US '(OUR CALL SIGN); YOU WERE NOT CLRED FOR TKOF!' THERE WAS NO CONFLICT WITH THE LIGHT ACFT. WE WAITED IN POS AND IN A MIN WERE CLRED FOR TKOF. THAT ONE WAS UNEVENTFUL. CONTRIBUTING FACTORS: TWR LAUNCHED 2 ACFT IN FRONT OF US WHILE IN POS ON RWY. WE SAW AND EXPECTED THE FIRST ACFT WHICH WAS CLOSER TO US AND CLRED FOR TKOF AS WE WERE CLRED INTO 'POS AND HOLD.' TWR DID NOT ADVISE US THAT HE WAS LAUNCHING A SECOND ACFT IN FRONT OF US. WE WERE 'EXPECTING' THE NEXT XMISSION TO BE 'OUR' TKOF CLRNC. UPON OUR READBACK OF WHAT WE ASSUMED WAS 'OUR' TKOF CLRNC; TWR SAID NOTHING (POSSIBLE COVERED XMISSION BY OTHER ACFT WHICH TOOK THE CLRNC)? DISTRS OF A) DOING LAST FEW ITEMS ON TKOF CHKLIST. B) WATCHING HIGH PERFORMANCE TKOF/TURNOUT BY FIRST ACFT TO LAUNCH IN FRONT OF US. TWR WAS PROBABLY TRYING TO GET AS MANY ACFT OFF ON 8R AS HE COULD BEFORE INBOUND TFC FOR 26L. (NOTE: 26R-8L CLOSED.) CORRECTIVE ACTIONS: I DON'T KNOW IF IT IS POLICY; BUT I LIKE TO BE TOLD WHEN ACFT WILL BE LAUNCHED ON A RWY I AM CLRED ONTO WITH MY ACFT IN POS. I'LL HAVE TO LISTEN MORE ATTENTIVELY FOR MY CALL SIGN; NOT ASSUME THAT THE NEXT CALL IS FOR MY ACFT. TWR CTLR SHOULD BE AWARE THAT ANY ACFT PUT IN 'POS AND HOLD' WILL PROBABLY BE EXPECTING TKOF CLRNC NEXT. TWR SHOULD INFORM THAT ACFT (WHEN POSSIBLE) IF SOME OTHER OP WILL BE PRECEDING THE EXPECTED CLRNC. (WE DID EXPECT THE LAUNCH OF THE FIRST ACFT IN FRONT OF US; AS HE WAS CLRED WHILE WE WERE STILL TAXIING INTO POS AND HE WAS CLOSE ENOUGH TO US TO BE EASILY SEEN TAXIING OUT.) WE WERE NOT EXPECTING A SECOND ACFT TO BE CLRED OUT ONTO THE RWY HALF WAY DOWN THE RWY AFTER WE WERE POSITIONED! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES HE FEELS BIGGEST PROBLEM IS LACK OF ANY TA INDICATING WHAT SEQUENCE OFF DEP WAS BEING USED. NATURALLY FEELS HE WILL BE NEXT FOR TKOF WHEN ALREADY IN POS AND ACFT AHEAD HAS DEPARTED. COULD NOT SEE OTHER ACFT AT INTXN HALF WAY DOWN THE RWY. DID NOT HEAR ANY COM WITH THAT ACFT PRIOR TO TKOF CLRNC. THEY WERE THE ONLY OTHER ACFT AT THE RWY AS FAR AS THEY KNEW. THAT'S WHY THEY NEVER QUESTIONED THE TKOF CLRNC.