PLT OF BE35-G CROSSES HOLD LINE AT LUB.

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I WAS TAXIING S ON TXWY A; TOWARDS THE OVERRUN OF RWY 8/26 WHERE IT CROSSED RWY 17/35. THE DAY EARLIER I HAD BEEN CLRED S ON TXWY A TO PARKING. TAXIING S ON TXWY A; I HAD TO CROSS THE HOLD LINES; WHICH I DID; AND NOTHING WAS SAID ABOUT XING THE HOLD LINES. I WONDERED ABOUT THAT; SINCE I WAS NOT SURE WHERE RWY 8/26 REALLY STARTED. THE NEXT NIGHT; WHEN I RECEIVED TAXI INSTRUCTIONS; I TAXIED TO THE HOLD LINES AND STOPPED. I WAS FACING A HVY TWIN AT THE OPPOSITE HOLD LINES AND THEY WERE CLRED TO DEPART AND I WAS TO BE THE NEXT DEP. WHEN I CALLED LUBBOCK TWR AND RECEIVED INSTRUCTIONS TO HOLD SHORT; I THOUGHT THAT THERE MIGHT BE ANOTHER SET OF HOLD SHORT LINES AT THE RWY 17/35 AND RWY 8/26 INTXN OR SOMEWHERE BTWN MY POS AND THE START OF RWY 8/26 -- ESPECIALLY SINCE THE YELLOW TAXI LINE RAN STRAIGHT AHEAD. WHEN I TAXIED AHEAD AND TURNED E; TWR ASKED ME IF I WAS ON RWY 8/26 AND I REPLIED NO; SINCE I DIDN'T SEE ANY WHITE RWY LINES AND I HAD TAXIED THERE ON THE PRIOR DAY WITH NO PROBS. TWR TOLD ME I WAS ON THE RWY AND CLRED ME TO DEPART. THERE WAS NO CONFLICTING TFC. I THINK THE RWY NEEDS TO BE MARKED WITH WHITE LINES -- ESPECIALLY WHERE IT IS ALSO PART OF A TXWY. I SHOULD HAVE ALSO ASKED FOR CLARIFICATION FROM THE TWR. LATE NIGHT DEP AND GENERAL FATIGUE PROBABLY ALSO CONTRIBUTED TO JUST GOING AHEAD AND TAXIING WITHOUT ASKING FOR CLARIFICATION WHEN IN DOUBT.