B737 FLC FAILED TO HOLD SHORT OF RWY AS INSTRUCTED IN TAXI OUT CLRNC DUE TO A MINOR ROUTING CHANGE PRIOR TO THE HOLD SHORT LEFT THEM TO BELIEVE THAT THE UPCOMING RWY HOLD SHORT NO LONGER APPLIED.

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ON PUSHBACK FROM GATE; LEG #2 OF 3 ON A 3-DAY TRIP; AFTER COMPLETING THE PRETKOF CHKLIST; WE TRIED TO CALL FOR TAXI CLRNC SEVERAL TIMES. THE GND CTLR WAS VERY BUSY AND SEEMED TO BE GETTING FRUSTRATED. WE FINALLY RECEIVED OUR CLRNC WHICH WAS TO TAXI TO RWY 4R VIA TXWY Y; HOLD SHORT OF RWY 31C. ON TAXI OUT; MY ATTN WAS ON ALL THE GND TFC MOVING ABOUT US AND ALSO ON THE OBSTRUCTION DUE TO THE RAMP CONSTRUCTION IN PROGRESS. AS WE APCHED TXWY Y; A COMPANY ACFT HAD JUST LANDED ON RWY 4R AND WAS CLRING THE RWY ON TXWY P. GND THEN TOLD US TO USE RWY 31R TO TXWY Y; TO ALLOW OUR COMPANY ACFT TO HIS GATE. WE THEN TURNED ONTO TXWY Y AND CONTINUED OUR TAXI. AS WE PASSED RWY 31C; THE GND CTLR ASKED US TO READ BACK OUR TAXI CLRNC; WHICH WAS TAXI TO RWY 4R; HOLD SHORT RWY 31C. AT THAT POINT I REALIZED I HAD MADE A MISTAKE. ALL ALONG IN MY MIND; I KEPT THINKING RWY 31R INSTEAD OF RWY 31C THE ACTIVE RWY THIS DAY WAS RWY 4R. SUPPLEMENTAL INFO FROM ACN 558927: I BELIEVE THE GND CTLR'S WORKLOAD AND OBVIOUS FRUSTRATION CONTRIBUTED TO THIS INCURSION INCIDENT BY CAUSING A RELUCTANCE TO QUESTION HIS INSTRUCTIONS WHEN HE WAS OBVIOUSLY OVER TASKED. HOWEVER; LOOKING BACK ON THE EVENT; STRICTLY FOLLOWING THE INSTRUCTIONS ACTUALLY GIVEN WOULD HAVE PREVENTED THIS INCIDENT. TO PREVENT THIS FROM HAPPENING IN THE FUTURE; I/WE CAN'T LET A CTLR'S PROBS BECOME OUR PROBS. WHEN CLRNC INSTRUCTIONS GIVEN ARE NOT CLR; I/WE NEED TO GET CLARIFICATION BEFORE CONTINUING; EVEN AT THE EXPENSE OF WAITING FOR THE CTLR'S WORKLOAD TO DIMINISH; OR ADDING TO THE WORKLOAD. THE CONSEQUENCE IS TOO IMPORTANT.