AN CARGO F27 FLC INADVERTENTLY TAXIED PAST THE HOLD SHORT AREA FOR RWY 10L AT PDX. THE FLC WAS DISTR BY AN ACFT TAXIING NEARBY AND MISSED ALL CUES THAT WOULD HAVE CAUSED THEM TO HOLD SHORT.

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PDX GND CTLR ISSUED CLRNC TO TAXI FROM THE RAMP TO RWY 10L VIA TXWY A TO TXWY A5 TO TXWY T (MAIN TERMINAL RAMP AREA). WHILE REACHING THE COMPLICATED INTXN OF TXWYS T AND A; THE CAPT AND I WERE DISTR BY A B737 TAXIING IN CLOSE FROM OUR L. AFTER SOME DISCUSSION WE DETERMINED WE HAD THE RIGHT OF WAY. WE DID NOT NOTICE THE LIGHTED RWY IDENT BOX USUALLY ASSOCIATED WITH THE HOLD SHORT LINE AT TWR CTLED ARPTS. AFTER TURNING R OFF TXWY T AND PROCEEDING OUT TO RWY 10L; WE DETERMINED THAT WE HAD TAXIED OVER THE HOLD SHORT LINE FOR RWY 10L BY APPROX 40 FT. THERE WAS NO TFC ON FINAL AND WE STOPPED. WE WERE 100 FT FROM RWY 10L. THE HOLD SHORT LINE IS USED FOR BOTH RWYS 10L AND 21; IN OTHER WORDS THERE IS ONLY ONE HOLD SHORT LINE FOR TWO RWYS; THIS CONFUSED US. AT THE TIME WE STOPPED WE WERE CALLED BY GND CTL AND ADVISED THAT WE WERE INDEED OVER THE HOLD SHORT LINE. NO TFC CONFLICT WAS CAUSED. NO HAZARD TO OTHER ACFT. CONTRIBUTING FACTORS: UNFAMILIARITY WITH ARPT; FATIGUE; 4 1/2 HRS LATE; CONSTRUCTION ON TXWY A RTE TO RWY 10L; REQUIRING A DETOUR TO TXWY T; AND A POORLY MARKED HOLD SHORT LINE RUNNING DIAGONAL ACROSS THE INTXN OF SEVERAL TXWYS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS RPTR HAS BEEN BACK TO PDX; AND IS NOW FAMILIAR WITH THE CONCEPT OF A DIAGONAL HOLD LINE FOR TWO RWYS. THIS FO STILL TAKES ISSUE WITH THE FACT THAT THIS IS AN UNCOMMON TYPE OF HOLD SHORT LINE AND THE ADJACENT RWY HOLDING POS SIGN IS TOO FAR AWAY FROM THE INTXN AND UNNOTICEABLE. THIS FLC HAD BEEN FLYING ALL NIGHT; BUT THIS RPTR FEELS THAT THIS WAS A MINOR FACTOR IN THE INCIDENT. THE COMBINATION OF THE DISTR WITH THE B737 JUST AT THE TIME THAT THE ACFT APCHED THE RWY HOLDING POS MARKING; WAS THE MAIN CAUSE OF THE CREW'S INABILITY TO HOLD SHORT.