B737-300; THINKING THEY WERE ON TXWY J; USED TXWY T INSTEAD OF TXWY J AND THEREFORE CROSSED RWY 27R WITHOUT CLRNC.

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WE TAXIED OUT OF CONCOURSE B TO SPOT Z. AT SPOT Z I CALLED GND CTL FOR TAXI. HE ISSUED INSTRUCTIONS; RWY 27L VIA TXWYS J; Y; AND S. I WROTE THIS DOWN. SPOT Z SITS AT THE INTXN OF TXWY T AND TXWY J. THE CAPT IMMEDIATELY ASKED ME TO CONFIRM NO HOLD SHORT INSTRUCTIONS IN THAT CLRNC. I CONFIRMED THAT YES; THERE WERE NO HOLD SHORT INSTRUCTIONS. (AT PHL THEY ALWAYS TELL YOU TO HOLD SHORT OF RWY 27R ON THE FIRST CALL). THE CAPT PUT THE PWR UP RIGHT AWAY TO CROSS RWY 27R AS HE CLRED THE RWY VISUALLY. SINCE HE HAD ASKED THE QUESTION; A RED FLAG WENT UP IN MY MIND AS TO OUR CLRNC. I LOOKED UP AT MY WRITTEN INSTRUCTIONS TO CONFIRM NO HOLD SHORT INSTRUCTIONS AND EVEN LOOKED UP OVER THE GLARE SHIELD TO CONFIRM TXWY J. I SAW THE SIGN FOR ONLY TXWY J RIGHT IN FRONT OF US. I FURTHER CLRED THE RWY (XCOCKPIT); TO ENSURE WE HAD A CLR PATH TO CROSS RWY 27R. I NOTED A COMPANY B737-200 CLRING THE RWY AT THE HIGH SPD TXWY K4. I ALSO NOTED ANOTHER PLANE ON FINAL A COUPLE OF MI OUT. IT LOOKED SAFE TO ME. WE CROSSED THE RWY IN AN EXPEDITIOUS MANNER. UPON OUR ARR ON THE OTHER SIDE OF RWY 27R I LOOKED DOWN AT MY ARPT DIAGRAM SINCE I DID NOT SEE ANY MORE TXWY J SIGNS ON THE OTHER SIDE. THE CAPT WAS DOING THE SAME. ABOUT THE SAME TIME WE BOTH CAME TO THE CONCLUSION THAT TXWY J GOES AROUND THE RWY AT THE DEP END; AND WE HAD CROSSED THE RWY AT TXWY T. HENCE; NO HOLD SHORT INSTRUCTIONS. WE TAXIED TO #1 FOR RWY 27L WITHOUT ANY CONVERSATIONS WITH GND OR TWR ABOUT THE ERROR.