E140 CREW ENTERED RWY 12R AT STL AS THEY BEGAN TAXI FROM SPOT H.

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ON MAR/WED/03; XA30 PM; I WAS TAXIING AN EMB140 FOR TKOF AT ST LOUIS LAMBERT AIRFIELD. I WAS HOLDING ON SPOT H WHEN GND TOLD ME TO TAXI TO RWY 12L VIA TXWY A AND HOLD SHORT OF RWY 12R AT TXWY E. IT WAS NIGHT AND RAINING. THIS AREA NEAR INTXN 'H' AND 'A' IS POORLY MARKED AND HAS POOR LIGHTING. AS I PULLED OFF SPOT H; I CALLED FOR OUR TAXI CHKLIST. TXWY A AND THE RAMP ARE ESSENTIALLY ONE LARGE TARMAC IN THIS AREA; MAKING IT DIFFICULT TO RECOGNIZE TXWY A. I TAXIED STRAIGHT ON TXWY H PAST TXWY A AND AS I WAS XING THE HOLD SHORT LINE; I NOTICED AN MD80 ON FINAL AND REALIZED THAT I WAS TAXIING ONTO THE RWY AND NOT TXWY A. I IMMEDIATELY INITIATED A 180 DEG TURN AND EXITED THE RWY AT SPOT H WHERE I ENTERED IT. AT THIS TIME THE MD80 INITIATED A GAR. SUPPLEMENTAL INFO FROM ACN 576101: I WAS INSTRUCTED TO START THE TAXI CHKLIST. WHILE READING THE TAXI CHKLIST I GLANCED UP TO SEE WE WERE ENTERING THE ACTIVE RWY 12R AT TXWY H. AT THAT SECOND; MY CAPT TURNED THE AIRPLANE OFF THE RWY AT THE SAME PLACE WE ENTERED. AS WE WERE TURNING OFF THE RWY THE LNDG TFC WAS GOING AROUND. THERE WAS AT LEAST 8000 FT OF SEPARATION. AFTER DISCUSSING THE INCIDENT WITH MY CAPT; I HAVE DECIDED THAT THE TIMING FOR THE TAXI CHKLIST AND MY CAPT'S COMFORTABILITY AND COMPLACENCY AT LAMBERT WERE THE REASON WE ENDED UP IN THE SIT WE DID.