MD80 CREW INADVERTENTLY TAXIED PAST THE HOLD SHORT LINES FOR RWY 25R AT TXWY B IN LAS.

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WE WERE CLRED TO TAXI TO RWY 25R. APCHING THE END OF TXWY B; WE TURNED THE CORNER AND HELD OUR POS ON THE TXWY SHORT OF THE RWY. AFTER WE HAD STOPPED; IT SEEMED AS IF WE WERE CLOSER THAN NORMAL TO THE RWY. WE WERE THEN CLRED FOR TKOF; AND WHEN I TURNED ON THE GND FLOOD LIGHTS; I COULD NOT SEE THE HOLD SHORT LINE -- EITHER IN FRONT OF THE ACFT OR OUT THE SIDE WINDOW ABEAM US. WE HAD INADVERTENTLY CROSSED THE HOLD SHORT LINE BEFORE WE STOPPED ON THE TXWY AWAITING CLRNC ONTO THE RWY FOR TKOF. WE DEPARTED AND HEARD NO COMMENTS FROM ATC. NO TFC LANDED ON RWY 25R DURING THIS ENTIRE PERIOD. WE SIMPLY DID NOT SEE THE HOLD SHORT LINE; BUT I BELIEVE THAT THERE WERE A COUPLE OF CONTRIBUTING FACTORS. I DO NOT RECALL SEEING ANY SIGNS OR TXWY LIGHTING IDENTING THE HOLD SHORT LINE. IT IS NOT IDENTED ON THE COMMERCIAL CHART ARPT PAGE. THIS ALSO OCCURRED DURING THE MIDDLE OF THE NIGHT; AND FATIGUE MAY HAVE PLAYED A ROLE. HAD THE HOLD SHORT LINE BEEN IDENTED WITH A SIGN OR LIGHTING THIS WOULD NOT HAVE OCCURRED. IN THE INTEREST OF SAFETY; EITHER THE LIGHTS OR SIGN SHOULD BE INSTALLED.