AN ACR CREW TAXIES INTO POS AND HOLD WITHOUT PROPER CLRNC CAUSING A GAR BY ARRIVING ACR.

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WE WERE TAXIING OUT AT STL TO RWY 30L. WE WERE E BOUND ON TXWY D. I WAS HOLDING SHORT OF TXWY S SO ACFT Y ON TXWY A COULD TAKE THE RWY IF APPLICABLE. THERE WAS ACFT Z; ON TXWY S HOLDING SHORT OF RWY. WE WERE IN A POSITION TO BE NEXT OR THE ACFT HOLDING ON TXWY A. AS WE WERE HOLDING WE HAD TO CALL FOR OUR TKOF NUMBERS AND THEY ARRIVED ON THE PRINTER. THE TWR TOLD ACFT W TO TAXI INTO POSITION. AT THIS POINT I LOOKED AROUND FOR A MD80 OR 757 BECAUSE THAT IS WHAT IS NORMALLY USED TO FLY FLT XXX. THERE WERE NO 757 OR MD80 ACFT VISIBLE. TO THE BEST OF MY KNOWLEDGE 717'S NEVER GO THERE. THE TWR CLRED FLT XXX FOR TKOF. I NEVER DID SEE FLT XXX TKOF. MAYBE HE USED AN INTXN OR MAYBE THE TWR WAS USING FREQ 118.5 FOR BOTH RWYS; I DON'T KNOW. THE FO AND I HAD A BRIEF DISCUSSION ABOUT THIS. I FELT LIKE I WAS JUST TRYING TO STAY IN THE LOOP OF THINGS. DURING THIS SAME TIME FRAME THE 717; ACFT Z; WAS CLRED FOR TKOF. THE TWR ASKED US IF WE WERE READY. WE WERE COMPLETING OUR TAXI CHKLIST AND I TOLD THE FO TO TELL THEM WE WERE READY. I ALSO NOTICED THAT IT WAS GETTING DARKER TO THE W AND I FELT LIKE THE SOONER WE GOT GOING THE BETTER. TO MY BEST RECALL; THE TWR TOLD US TO TAXI UP TO THE NUMBER POSITION AND HOLD SHORT. I SAID POSITION AND HOLD. I LOOKED TO THE E AND I DID NOT SEE ANYONE ON FINAL. I TURNED ONTO TXWY S AND WENT INTO TKOF POSITION ON RWY 30L AND HELD. AS WE WAITED IN POSITION; I THINK SOMEONE CROSSED THE RWY DOWN FIELD; BUT I AM NOT SURE. THE NEXT THING THAT HAPPENED WAS FLT YYY; ACFT V; ANNOUNCED HE WAS GOING AROUND. THIS CAME FROM FLT YYY NOT THE TWR. ALL THE TIME WE WERE IN POSITION; THE TWR APPARENTLY WAS NOT AWARE OF IT. AT THIS POINT THE TWR ASKED US WHY WE WERE THERE. THE FO TOLD THE TWR WE MADE A MISTAKE. AFTER A SHORT PERIOD WE WERE CLRED FOR TKOF. SEVERAL FACTORS LED TO THIS SIT. PREOCCUPATION WITH WX; CHKLISTS; AND AN UNKNOWN ACFT ON RWY 30L THAT I HEARD BUT DID NOT SEE. PERHAPS I HEARD WHAT I WANTED TO HEAR--POSITION AND HOLD. IT SEEMED LIKE THE LOGICAL SEQUENCE AT THE TIME WHEN THE TWR ASKED US IF WE WERE READY. I HAVE REEVALUATED MY ACTIONS AND HAVE DECIDED TO MAKE THE FOLLOWING CHANGES TO MY COCKPIT OP. I WILL MAKE ALL COCKPIT CALLS LOUDER AND BE EVEN MORE AWARE OF COMPOUNDING DISTRACTIONS THAT CAN HAVE A DETRIMENTAL EFFECT ON THE OP.