FLC CROSSES HOLD LINE AT STL.

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DURING TAXI (PREFLT) WE WERE HOLDING AT TXWY N AND TXWY A WAITING FURTHER TAXI CLRNC. WE WERE GIVEN WHAT WE THOUGHT WAS CLRNC TO CROSS RWY 30L AND HOLD SHORT AT TXWY R. I READ BACK THE CLRNC TO STL GND CTL AND BOTH THE CAPT AND I ASSUMED HE MEANT HOLD SHORT OF RWY 30R. AT THE TIME; A DC9 WAS TAXIING ONTO RWY 30L; BUT HAD NOT BEEN CLRED FOR TKOF. MY CAPT ASSUMED THAT GND WANTED US TO EXPEDITE OUR TAXI ACROSS RWY 30L SINCE THERE WAS ANOTHER ACFT ON ABOUT A 4 MI FINAL. AS WE WENT TO CROSS THE RWY ON TXWY N; GND CTL TOLD US TO STOP AT OUR CURRENT POS SHORT OF RWY 30L. ALTHOUGH OUR ACFT MAY HAVE CROSSED THE YELLOW HOLD SHORT LINE; THE ACFT WAS STOPPED WELL SHORT OF RWY 30L. NO RWY INCURSION OCCURRED AND DURING THIS TIME; THE ACFT ON FINAL INITIATED A GAR WITH THE DC9 HOLDING IN POS AT RWY 30L. WE WERE THEN CLRED TO CROSS RWY 30L AND PROCEED TO TAXI TO RWY 30R. GND INFORMED THE CAPT TO CALL THE STL TWR SUPVR TO DISCUSS WHAT HAPPENED. APPARENTLY; GND CTL WANTED US TO TAXI TO TXWY R AND HOLD SHORT OF RWY 30L. THIS WOULD HAVE REQUIRED US TO TAXI E ON TXWY A; HOWEVER; THAT WAS NOT INCLUDED IN THE INSTRUCTION. ALTHOUGH I HONESTLY CANNOT REMEMBER VERBATIM OUR TAXI CLRNC OR MY READBACK; GND CTL; AFTER LISTENING TO THE TAPE REPLAY; CONCLUDED THAT MY TAXI INSTRUCTION READBACK WAS CORRECT. HOWEVER; FOR WHATEVER REASON; BOTH THE CAPT AND I UNDERSTOOD TO CROSS RWY 30L AND HOLD SHORT OF RWY 30R AT TXWY R. I BELIEVE THAT 2 THINGS WOULD HAVE PREVENTED THIS MISUNDERSTANDING. FIRST; A CLRNC TO 'TAXI E ON TXWY A TO TXWY R THEN HOLD SHORT OF RWY 30L' WOULD HAVE BEEN A MORE DESCRIPTIVE TAXI CLRNC. SECOND; ON OUR PART THERE SHOULD HAVE BEEN NO RUSHING TO TRY TO CROSS A RWY WHEN IT WAS OBVIOUS WE WOULD HAVE TO WAIT SEVERAL MINS ANYWAY FOR TKOF DUE TO LARGE NUMBERS OF ACFT ALREADY WAITING IN LINE. IT WOULD HAVE BEEN MUCH BETTER FOR US TO SIMPLY HOLD OUR POS AND WAIT FOR FURTHER CLRNC SINCE A DELAY WAS INEVITABLE ANYWAY.