A B737-400 FLC TAXIED ONTO A SEA RWY WITHOUT CLRNC AND FORCED ANOTHER ACFT TO GO AROUND.

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TAXIING FULL LENGTH RWY 34R. GND CTL DIRECTED US TO MOVE TO THE L SIDE OF THE TXWY AND PASS ACFT GOING TO RWY 34R AT INTXN 1. AS WE APCHED THE END OF THE TXWY TO RWY 34R FULL LENGTH; TWR CLRED US INTO POS AND HOLD. AS WE GOT CLOSER; I DOUBLECHKED WITH THE FO THAT WE WERE CLRED INTO POS. THE FO SAID YES. AS WE ROUNDED THE CORNER ON THE ROLL; WE SAW TFC ON FINAL. THE FO ASKED TWR TO CONFIRM 'ACFT 123 CLRED INTO POS AND HOLD.' TWR RESPONDED; 'ACFT 123 (HESITATION) IS CLRED INTO POS AND HOLD.' WE FELT THAT WE WOULD BE CLRED IMMEDIATELY FOR TKOF AND WOULD LIFT OFF PRIOR TO THE DASH 8 TFC TOUCHING DOWN; SO WE FOLLOWED INSTRUCTIONS AND WENT AND THEN REMAINED IN POS. AS WE LINED UP WE SAW B737 TFC THAT APPEARED STATIONARY AT RWY 34R INTXN Q. LATER WE SAW THIS TFC ROLL AND CLR THE RWY. I HAD THOUGHT OF CLRING THE RWY AS I FIRST TURNED ON; BUT DID NOT HAVE TIME TO HAVE THE FO VERIFY THAT A U-TURN ON THE TXWY WAS AVAILABLE AS FAR AS TFC BEHIND US ON THE TXWY WAS CONCERNED. I DID NOT WANT TO RISK TURNING OFF THE RWY OR MAKING A U-TURN WITHOUT FIRST CLRING IT FOR TFC. WE KNEW TWR WAS AWARE OF OUR POS; AND VISIBILITY WAS GOOD SO THE APCH TFC COULD ALSO SEE US. POSSIBLE CORRECTION; ALTHOUGH WE WERE TAXIING AT A NORMAL TAXI SPD; 1) TAXIING SLOWER AND TAKING AN EARLIER AND LONGER LOOK AT TFC ON FINAL WOULD MAKE IT EASIER TO DETERMINE DISTANCE OF TFC ON FINAL. 2) WE THOUGHT THAT WE WERE COMPLYING WITH TWR'S INSTRUCTIONS AND WE STILL DO; BUT IT IS POSSIBLE THAT WE HEARD THE CLRNC WRONG AND TWR DID NOT HEAR AND CORRECT OUR READBACK. I FEEL READBACK IS VERY IMPORTANT TO THE SAFETY OF FLT. WE THE CREW THOUGHT WE DID EVERYTHING RIGHT AND VERIFIED THE CLRNC 2 TIMES; BUT SOMEHOW CONFUSION STILL EXISTED BTWN TWR AND US. IT WOULD BE EASY TO MIX OUR FLT NUMBER UP (123) WITH SOMEONE ELSE'S FLT NUMBER (ACFT 423) BUT DON'T THINK THIS IS THE CASE. I GUESS THIS IS WHY THERE ARE 2 IN OUR COCKPIT. I'M SURE AT LEAST A COUPLE CTLRS; AND 2 IN THE COCKPIT OF THE OTHER ACFT ALL WATCHING AND LISTENING TO PREVENT AN ACCIDENT. SUPPLEMENTAL INFO FROM ACN 453630: WE BOTH (CAPT AND FO) HEARD THE POS AND HOLD INSTRUCTION; AND REPLIED WITH OUR CALL SIGN. WE AGAIN CALLED TO VERIFY AS WE APCHED THE RWY AND SAW TFC ON 2 MI FINAL. WE SURMISED TWR WANTED US TO DEPART IN FRONT OF THE DASH 8; UNTIL WE SAW A B737 IN POS AT INTXN Q; 3000 FT DOWN THE RWY. WE ARE ASSUMING THE CTLR FILED A RPT ON US.