AN ACR CREW TAXIES ACROSS AN INACTIVE RWY WITHOUT CLRNC WHEN THEY BEGIN TAXI TO THE USUAL RATHER THAN THE ASSIGNED RWY FOR DEP.

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PUSHED BACK FROM GATE IN BWI. RECEIVED TAXI INSTRUCTIONS TO RWY 15R; TURNED EAST (RIGHT). AFTER RECEIVING INSTRUCTIONS THAT ACR Y WAS HOLDING FOR ME; BEGAN TO HURRY. FELL INTO NORMAL TAXI TO RWY 28 VIA 'C' TXWY AND CROSSED 4-22 RWY; REALIZED MISTAKE AND CLARIFIED TAXI ROUTE TO RWY 15R. JUMPSEAT WAS OCCUPIED. WE WERE HAVING CONVERSATION ABOUT ACR OPS. HE ASKED AS I WAS TAXIING OUT; 'AREN'T WE USING RWY 15R?' AS I WAS CROSSING RWY 4 ON TXWY 'C.' I HAD FAIRLY NEW FO WHO WAS BUSY. LOTS OF CHATTER ON RADIO; I MESSED UP! FELL INTO HABIT PATTERN TO RWY 28; HEARD WHAT I WANTED TO HEAR -- TAXI TO CLOSEST RWY. I DID LOOK UP AND DOWN RWYS 4-22 VERY CAREFULLY; AS AT THIS TIME THINGS WEREN'T FEELING RIGHT; BUT WAS TOO LATE JUST TO STOP. THERE WAS NEVER A SAFETY HAZARD AND I REALIZED I MADE AN UNINTENTIONAL MISTAKE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CALLBACK REVEALED THAT THE TAXI CLRNC WAS 'TAXI TO RWY 15R' WITH NO ROUTING INFO PROVIDED AS TO TAXIWAYS TO BE USED OR DIRECTION TO TAXI. THE JUMPSEAT RIDER IN THE COCKPIT WHO NOTICED THE ERROR WAS LOCALLY BASED. HE WAS WEARING A HEADSET AND WAS NOT CONSIDERED A DISTR BY THE RPTR; BUT AN ASSET TO THE OP. THE GND CTLR APPEARED TO HAVE A HEAVY WORKLOAD AND WAS NOT AWARE OF THE TAXI ERROR UNTIL NOTIFIED BY THE CREW. THE OTHER ACFT INVOLVED WAS ONLY HOLDING WAITING FOR THE RPTR TO VACATE THE GATE AREA. THERE WAS NO CONFLICT.