FLC OF A DEHAVILLAND DASH 8-100 CROSSED THE HOLD SHORT LINE WHEN TAXIING FOR TKOF DUE TO NOT OBSERVING THE HOLD SHORT LINE DUE TO BELIEVING THAT IT WAS FOR ANOTHER RWY TXWY INTXN.

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REQUESTED TAXI TO ACTIVE RWY. WE WERE CLRED TO RWY 33 AT ROA; WHICH WE WERE UNABLE TO ACCEPT DUE TO A WT RESTR. ATC THEN CLRED US TO RWY 6; HOLD SHORT AT RWY 33 AND INTXN D; WHICH WE ACCEPTED. WE WERE THEN CLRED TO CROSS RWY 33 AND OUT TO RWY 6. AS WE WERE TAXIING DOWN TXWY E; I NOTICED A RWY HOLD SHORT LINE WHICH I BELIEVED TO BE THE HOLD SHORT LINE AT TXWY J AND RWY 6; AND CONTINUED ON DOWN TO THE BEGINNING OF RWY 6. AS WE APCHED THE BEGINNING OF THE RWY; GND CTL CONTACTED US TO ADVISE THAT WE HAD MISSED THE HOLD SHORT LINE. THEY INSTRUCTED US TO CALL THE TWR UPON OUR ARR IN PHL. WHEN I CONTACTED THE TWR IN ROA; I SPOKE TO TWR SUPVR WHO ADVISED ME THAT HE WAS FILLING OUT A PLTDEV RPT AND REQUESTED PLT/ACFT INFO. HE THEN WENT ON TO TELL ME THAT THIS PARTICULAR INCURSION HAD OCCURRED ON NUMEROUS OCCASIONS AND AGREED THAT IT IS SOMEWHAT CONFUSING. HE TOLD ME THAT WHEN USING RWY 6; GND CTL WAS TO ADVISE THE PLT THERE WAS A DISPLACED THRESHOLD AND THAT FLASHING LIGHTS ALSO SHOULD BE TURNED ON AT THE RWY 6 HOLD SHORT LINE. WE DID NOT RECEIVE THE DISPLACED THRESHOLD WARNING ON THIS OCCASION AND NEITHER THE FO NOR I RECALL THE FLASHING LIGHTS AT THE HOLD SHORT LINE. THE REASON I SUSPECT THE OMISSION IS BECAUSE RWY 6 WAS NOT THE PREFERRED ACTIVE RWY. ROA WAS ALSO USING RWY 33 AND RWY 21.