AN MD80 CREW FEELS A POSSIBLE RWY INCURSION OCCURRED DURING TAXI FOR TKOF.

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WE WERE TAXIING TO RWY 35R IN SDF ON THE PARALLEL; TXWY D. IT WAS DAY; VFR AND THE ARPT WAS NOT BUSY AS WE COULD TELL WE WOULD BE #1 FOR DEP AS WE LEFT THE GATE. AFTER XING RWY 11/29 ON TXWY D; THE CAPT CALLED FOR THE BEFORE TKOF CHKLIST. MY ATTN WENT FROM OUTSIDE THE ACFT TO INSIDE FOR CHKLIST COMPLETION. I SWITCHED OUR RADIO TO TWR FREQ AT GND'S REQUEST. AFTER COMPLETING THE CHKLIST; I LOOKED OUTSIDE AND SAW THE RED SIGN 'RWY 35R-APCH' AND RWY HOLD LINE RAPIDLY APCHING. BEFORE I COULD TAKE ACTION (EITHER VERBALLY OR ACTIVELY) IN STOPPING THE AIRPLANE; TWR CLRED US ONTO THE RWY FOR TKOF. IF TWR HAD NOT CLRED US TO TAKE OFF AT THAT EXACT MOMENT; A RWY INCURSION WOULD CERTAINLY HAVE OCCURRED DUE TO OUR SPD APCHING THE HOLD LINE. THERE IS SOME CONFUSION; ESPECIALLY AT LOUISVILLE; WITH REGARD TO MANDATORY RWY HOLDING SIGNS (RWY 35R) AND MANDATORY RWY APCH AREA HOLDING SIGNS (RWY 35R-APCH). MAINTAINING OUTSIDE VIGILANCE AS WELL AS GND/TWR SPECIFIC INSTRUCTIONS WOULD MOST CERTAINLY RECTIFY THIS SIT IN THE FUTURE.