AN ACR WDB TAXIED ACROSS THE END OF RWY 28C AT PIT WITHOUT CLRNC.

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WE LANDED ON 28L AT PIT AND TURNED OFF THE ACTIVE RWY ON HIGHSPD TAXIWAY C-3. ONCE CLR OF THE RWY WE CONTACTED GND CTL. ATC CLRED US VIA CHARLIE TO THE RAMP. ATC SHOULD HAVE NOTICED THAT WE WERE ALREADY PAST CHARLIE TAXIWAY. IN THE CONFUSION OF TRYING TO FIND CHARLIE TAXIWAY AND DETERMINE THE RTE; WE CONTINUED STRAIGHT AHEAD THINKING WE WERE ON THE CORRECT TAXIWAY. PRIOR TO XING 10C; I LOOKED R TO MAKE SURE THERE WAS NOTHING ON THE RWY. AFTER XING THE RWY; ATC ASKED; DO YOU KNOW YOU ARE NOT ON CHARLIE TAXIWAY AND THAT WE HAD CROSSED AN ACTIVE RWY. WE THEN REALIZED WE WERE ON THE WRONG TAXIWAY; ATC THEN CLRED US TO THE RAMP. ATC SHOULD HAVE LOOKED AND NOTICED WE HAD PASSED TAXIWAY CHARLIE. SHORT DISTANCE FROM HIGHSPD TAXIWAY TO ANOTHER ACTIVE RWY. TAXIING THE MOST PRACTICAL WAY TO THE RAMP. VERY POORLY MARKED AND ALL DIFFERENT NAMES AND NUMBERS FOR TAXIWAYS; AND GIVEN BY ATC AS IF YOU LIVE THERE. COMING OFF HIGHSPD TAXIWAY TRYING TO GET FREQ FOR GND; RAMP; CALLING COMPANY; AS WELL AS TRYING TO LISTEN; UNDERSTAND AND FIGURE OUT YOUR TAXI RTE.