RWY INCURSION DURING A NIGHT OP BY THE FLC OF A CARGO DC9 AT SWF; NY.

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WE WERE PARKED ON CARGO RAMP; N END OF TXWY M. WE WERE ISSUED INSTRUCTIONS; BY GND; TO 'TAXI TO RWY 9.' NO HOLD SHORT INSTRUCTIONS WERE GIVEN. I PROCEEDED TO TAXI PAST THE HOLD SHORT LINE AT THE INTERSECTION OF TAXIWAY M AND RWY 9. THE CONTROLLER ASKED; 'WHAT RWY ARE YOU GOING TO?' I CAME TO AN ABRUPT STOP WHICH LEFT THE NOSE OF MY ACFT FOUR FEET PAST THE HOLD SHORT LINE. MY FO STATED OUR POSITION. THE CTLR THEN CLEARED US ONTO RWY 9 TO BACK TAXI INTO POSITION AND HOLD. I ASKED THE CTLR IF THIS WAS GOING TO BE A PROBLEM. HE SAID; 'NO; JUST LISTEN UP.' THERE WAS NEVER ANY CONFLICT WITH ANY OTHER VEHICLE OR ACFT AT ANY TIME. THE WAY THE TAXI CLEARANCE WAS GIVEN; MY FO AND I MISTAKENLY MISUNDERSTOOD THE CLEARANCE TO MEAN THAT WE SHOULD TAXI ALL THE WAY TO THE APPROACH END OF RWY 9 SINCE NO HOLD SHORT INSTRUCTIONS WERE GIVEN. ADDITIONAL INFO FROM ACN 560337: BOTH THE CAPT AND I INTERPRETED THE GROUND CTLR'S TAXI CLEARANCE TO MEAN THAT WE SHOULD CROSS RWY 9 AND CONTINUE TO TAXI TO THE APP END OF RWY 9. THE CONTROLLER'S TAXI INSTRUCTIONS WERE VAGUE. HOWEVER; UPON REFLECTION; I WISHED THAT I HAD ASKED HIM TO CLARIFY.