CLRED TO HOLD SHORT; B737-800 FLT CREW EXPERIENCES RWY INCURSION AT COS; RWY 12 DEP END WHILE TAXIING TO RWY 17L VIA FOX TROT.

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I WAS THE FO DEPARTING COS. NEITHER I NOR THE CAPT WAS VERY FAMILIAR WITH THE ARPT. I HAD BEEN TO THE ARPT ONCE BEFORE BUT NEITHER OF US HAD PREVIOUSLY USED TXWY F. WE WERE HVY AND REQUIRED A NO-ENG-BLEED TKOF TO DEPART ON RWY 17L. AFTER PUSHING BACK FROM GATE; I STARTED BOTH ENGS IN THE RAMP AREA TO ALLOW TIME TO DO THE NO-ENG-BLEED TKOF CHKLIST. AFTER STARTING BOTH ENGS; WE RECEIVED A CLRNC TO TAXI TO RWY 17L VIA TXWY M; TXWY F; TXWY E; HOLD SHORT OF RWY 12. AFTER IDENTING THE RTE ON THE COMMERCIAL CHART PAGE AND THE CORRECT TXWY OUT OF THE RAMP; WE BEGAN TAXIING. I COMPLETED THE AFTER START CHKLIST WHILE IN THE RAMP AREA APCHING TXWY M; THEN COMPLETED THE TAXI CHKLIST ON TXWY M APCHING TXWY H. I BEGAN THE NO-ENG-BLEED TKOF CHKLIST ON TXWY M AT APPROX THE INTXN OF TXWYS M AND G. ON COMPLETION OF THE CHKLIST; I LOOKED UP AND SAW US XING AN INTXN AND SAID TO THE CAPT SOMETHING TO THE EFFECT OF 'HOLD SHORT OF THE RWY!' I WAS NOT SURE OF OUR LOCATION AND THERE WAS NO RWY SIGN IN SIGHT AT THE MOMENT. THE CAPT BRAKED AS WE LOOKED ABOUT TO CONFIRM OUR LOCATION BUT WE WERE ALREADY ACROSS THE RWY 12/30 HOLD SHORT LINE AND WE WERE ALMOST HALF WAY ACROSS THE RWY BEFORE WE WERE CERTAIN THAT IT WAS INDEED A RWY WHICH WE WERE XING. I IMMEDIATELY REACHED UP TO CALL TWR BUT THEY CALLED US BEFORE I KEYED THE MIKE. THEY CONFIRMED THAT THERE WAS NO CONFLICTING TFC AND CLRED US TO CONTINUE TO RWY 17L. FACTORS WHICH CONTRIBUTED TO THIS INCIDENT WERE: 1) OUR UNFAMILIARITY WITH COS ARPT; AND 2) THE NECESSITY TO COMPLETE AN ADDITIONAL CHKLIST FROM OUR OPS MANUAL WHICH REQUIRED HEADS-DOWN TIME READING THE MANUAL AND MOVING SWITCHES ON THE OVERHEAD PANEL; AND 3) THE APPEARANCE OF THE RWY INTXN. TXWY F CROSSES THE END OF RWY 12/30 OUTSIDE OF THE DISPLACED THRESHOLD. TXWY F IS A CONTINUOUS LIGHT-COLORED SURFACE WHILE THE DARK ASPHALT OF RWY 12 IS PAVED UP TO (NOT OVER) THE TXWY. THIS IS READILY APPARENT FROM OVERHEAD PICTURES. THE CAUSE OF THIS INCIDENT WAS OUR FAILURE TO MAINTAIN AWARENESS OF OUR LOCATION. I HAVE CHANGED MY TECHNIQUES TO PREVENT A RECURRENCE OF THE INCIDENT. I NOW WAIT UNTIL THE ACFT IS STOPPED BEFORE COMPLETING THE NO-ENG-BLEED TKOF CHKLIST. THIS MAY DELAY TKOF; BUT THE SAFETY-CONSCIOUS ATTITUDE AMONG CAPTS ALLOWS ME TO DO THIS WITHOUT CONFLICT.