CAPT OF AN MLG FAILED TO HOLD SHORT OF A RWY DURING TAXI FOR TKOF.

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I HAD JUST COMPLETED THE TRANSITION TRAINING TO THE F100. I HAD FLOWN THIS ACFT ALMOST 2 YRS BEFORE; SO THE TRAINING WAS ABBREVIATED; AND THE LAST 3 DAY TRIP WAS UNDER THE METICULOUS EYE OF A VERY DIFFICULT CHK PLT. I WAS FINALLY IN COMMAND AGAIN; TOTALLY RELAXED; THE FIRST LEG OF A 4 DAY PAIRING WITH A GOOD FO I HAD FLOWN WITH BEFORE. TOO RELAXED! WE TAXIED TO SPOT X PER RAMP CTL'S INSTRUCTIONS; AND CONTACTED GND CTL AS WE APCHED THE SPOT. THERE WERE NO ACFT ANYWHERE IN SIGHT; SO IT LOOKED LIKE WE WERE GOING TO GET RIGHT ON OUT. GND CTL SAID: 'CLRED TO RWY 27L VIA TXWY T; HOLD SHORT OF TXWY K.' WE WERE JUST TURNING THE CORNER ONTO TXWY T; APCHING RWY 27R WHEN THE FO READ BACK THE INSTRUCTIONS. I LOOKED BACK DOWN THE TXWY; AND SAW THE 'K' SIGN ABOUT 800 FT DOWN WHERE WE WOULD TAXI AFTER XING RWY 27R AND TURNING L AGAIN. I 'ASSUMED' THAT SINCE WE WERE ON TXWY T WE WOULD BE STOPPING AT TXWY K FURTHER DOWN THE ROAD WHERE I SAW THE TXWY K SIGN. WRONG! WE HAD ALREADY CROSSED TXWY K AND WERE PROCEEDING TO CROSS RWY 27R; WHERE; AS ALWAYS; I LOOKED BOTH WAYS AND RPTED 'RWY CLR L AND R.' THE FO WAS BURIED IN THE MANUAL; TRYING TO FIND TXWY K. AFTER WE WERE WELL CLR OF RWY 27R; WE WERE INFORMED THAT WE HAD CROSSED THE RWY WITHOUT CLRNC. THERE WERE NO ACFT CONFLICTS ANYWHERE; IN FACT; THE 'HOLD SHORT' INSTRUCTION WAS AN AUTOMATIC RESPONSE GIVEN FOR NO REASON AT ALL. NOT ONE OTHER ACFT WAS TAXIING OR LNDG. WE WERE WRONG TO PROCEED WITHOUT CLR ORIENTATION OF THE TXWYS. WE BOTH FELT THAT THE 'HOLD SHORT OF TXWY K' INSTRUCTIONS AS WE WERE XING TXWY K WERE GIVEN TOO LATE. THERE WAS NO REASON TO HOLD AT TXWY K; BUT 'HOLD SHORT OF RWY 27R AT TXWY K' WOULD HAVE BEEN LESS CONFUSING. PLUS THE NEW DESIGN OF TAXI SIGNS HAS NOT ONLY THE TXWY YOU'RE XING BUT THE TXWY YOU'RE ON; SO LOOKING BACK AT A 'K' SIGN DOWN THE ROAD DOES NOT MEAN THE TXWY IS DOWN THE ROAD ANYMORE. AFTER RETURNING TO BASE 4 DAYS LATER; I SAW A WORN SIGN ON THE UNION BULLETIN BOARD: 'UNION SAFETY ALERT' '2 PHL RWY INCURSIONS WITHIN THE LAST MONTH'!!