ACR HAS RWY INCURSION AT STL; MO.

Narrative

DURING OUR TAXI OUT AT STL; WE WERE INSTRUCTED TO 'TAXI TO RWY 12L; HOLD SHORT RWY 12R AT TXWY E.' WE ACKNOWLEDGED. THE FO WAS DOING THE TAXI CHK AND WAS INTERRUPTED BY GND WITH INSTRUCTIONS TO FOLLOW THE ATR72. WE COMPLETED THE TAXI CHKLIST AS WE PULLED UP BEHIND THE ATR ON TXWY E. THE GND CTLR ADVISED 'ACFT 3 MI OUT FOR RWY 12R;' THEN INSTRUCTED 3 ACFT (THE 2 ACFT AHEAD OF US AND WHAT WE PERCEIVED TO BE US) TO CROSS RWY 12R AND HOLD SHORT RWY 12L. WE ACKNOWLEDGED WITH 'CROSS RWY 12R; HOLD SHORT RWY 12L; ACFT X.' WE THEN STARTED TO FOLLOW THE OTHER 2 ACFT ACROSS RWY 12R. JUST AS WE WERE XING THE HOLD SHORT LINE; GND SAID 'ACFT X; WHAT ARE YOU DOING?' I STOPPED THE ACFT IMMEDIATELY JUST SHORT OF THE RWY. MY FO SAID 'WE WERE TOLD TO CROSS RWY 12R AND WE READ IT BACK.' THE GND CTLR SAID 'THAT WASN'T FOR YOU;' PAUSED; THEN SAID 'CROSS RWY 12R; HOLD SHORT RWY 12L.' AS I CROSSED RWY 12R; I SAW AN ACFT TURNING OFF THE APCH ABOUT 1 MI OUT. DURING OUR CLBOUT; THE CTLR ASKED US TO CALL STL TWR WHEN WE GOT A CHANCE AT HIS PHONE NUMBER. AFTER LEVELOFF AT FL350; THE FO MONITORED CTR AND THE ACFT WHILE I CALLED MY DISPATCHER ON THE FLT PHONE. HE CONNECTED ME WITH THE TWR SUPVR. HE SAID THEY HAD PULLED THE TAPES; AND THE CLRNC WE ACKNOWLEDGED WAS FOR ACFT Y; WHO WAS TO CROSS AT AN INTXN FURTHER DOWN THE RWY. OUR ACKNOWLEDGEMENT; HOWEVER; WAS CLRLY ON THE TAPE. ADMITTEDLY; THERE WAS A MISTAKE ON OUR PART AS WELL AS THE CTLR'S. CIRCUMSTANCES CONTRIBUTING TO THE SIT WERE: IT WAS RAINING. THE TEMP WAS -1 DEG C; SO I WAS TRYING TO SEE IF ANY ICE WAS FORMING. ALSO RUNNING OVER IN MY MIND WHAT I WOULD DO IN A 'REJECT' SIT. WHEN THE INSTRUCTIONS WERE GIVEN; I PERCEIVED IT TO BE FOR US. WE ANSWERED BACK LOUD AND CLR. NO OTHER CONVERSATION WAS GOING ON IN OUR ACFT; BUT A LOT OF INSTRUCTIONS WERE GIVEN IN A ROW; AND WE SIMPLY MISUNDERSTOOD. IN OUR DEFENSE; HOWEVER; THERE WAS AMPLE TIME FOR THE CTLR TO CORRECT OUR ACKNOWLEDGEMENT AND HE DID NOT. AS TO WHAT COULD BE DONE TO PREVENT A RECURRENCE OR CORRECT THE SIT: MORE VIGILANCE ON BOTH OUR PARTS -- OURS IN LISTENING MORE CLOSELY TO THE CTLR; AND HIS IN RESPONDING TO OUR ACKNOWLEDGEMENT.