UNAUTH RWY XING ENTRY OP. RWY INCURSION BY B727 FREIGHTER IN A NIGHT OP. POTENTIAL CONFLICT OCCURRED.

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I WAS TAXIING ON THE GND IN AUSTIN; TX. GND TOLD US TO HOLD SHORT OF RWY 35 AT TXWY G. WE WERE CLRED TO CROSS RWY 35; GO R ON TXWY H; R ON TXWY L; TAKE TXWY D-C TO RWY 31L. I CROSSED RWY 35 AND CONTINUED LOOKING FOR TXWY H. THE FIRST R TURN I SAW I THOUGHT WAS TXWY H. I WENT R AND REALIZED AT THAT MOMENT; I WAS ON RWY 35L. I HAD NOT SEEN ANY MARKING SIGNS FOR THE RWY. THE WHOLE CREW AND MYSELF SAW A TAXI LIGHT OF ANOTHER ACFT FACING US. IT LOOKED AS IF IT WAS ON THE ROLL TAKING OFF. THE ACFT WAS SO FAR FROM US THAT I HAD PLENTY OF TIME TO TAXI AHEAD TO THE NEAREST TURNOFF WHICH WAS TXWY L. I WENT R ONTO TXWY L. THE OTHER ACFT HAD DEPARTED RWY 31L SAFELY; IT WAS ABOUT 2000 FT IN FRONT WHEN IT TOOK OFF. I WAS LOOKING AT TAXIING OFF AT THE NEAREST TXWY. THE COPLT TOLD ME THAT THE ACFT WAS OFF THE GND AND CLRED US SAFELY. THE OTHER CREW MEMBERS FELT THE OTHER ACFT MIGHT HAVE BEEN ON A GAR. WHEN CLR OF THE RWY ON TXWY L; THE COPLT CALLED GND AND SAID WE ARE ON TXWY L. GND SAID; 'I HAVE A NEW CLRNC FOR YOU; ARE YOU READY TO COPY?' THE COPLT COPIED IT AND READ IT BACK. I CONTINUED TO TAXI TXWY L-D-C TO RWY 31L. AT RWY 31L THE COPLT CALLED TWR AND TWR CLRED US TO TKOF. NEITHER GND OR TWR SAID ANYTHING TO US ABOUT THE TAXIING CONFUSION WE HAD. THE OTHER ACFT AND WE BOTH TOOK OFF SAFELY AND THERE WAS NO ACCIDENT INVOLVED THROUGH THE CONFUSION OF TAXIING. I DIDN'T SEE TXWY H OR RWY 31L MARKINGS AND SINCE THE RWYS ARE SO CLOSE TOGETHER AT THIS POINT I FEEL THE GND CTL SHOULD HAVE POINTED THIS OUT CLRLY; IN ORDER TO PREVENT ANY CONFUSION IN TAXIING.