EMB145 FLT CREW TAXIES ACROSS A RWY AT ORD.

Narrative

WE WERE TAXIING OUT FROM THE GATE AND GOT AN INITIAL CLRNC TO GO THE 'PENALTY BOX' TO WORK OUT NUMBERS. DURING THE TAXI; WE FINISHED OUR NUMBERS AND TOLD GND; AFTER 5 MINS OF TRYING TO GET A BREAK IN GND'S XMISSIONS; WE WERE OK TO GO. THEY CLRED US VIA TXWYS A; H; P; TO WHAT I THOUGHT AT THE TIME WAS RWY 32L. I GLANCED AT THE TAXI CHART; FROM TXWY A TO TXWY P; AND LOOKED UP TO SEE TXWY P APCHING. SEEING A LINE OF OTHER PLANES HEADED TOWARDS RWY 32L; I THOUGHT I WAS CORRECT IN TAKING THE L ON TXWY P. WE TURNED; STOPPED AT THE THRESHOLD OF RWY 4L/22R; LOOKED ON BOTH SIDES; AND CROSSED. UPON REACHING THE OTHER SIDE; I REALIZED TXWY P WAS NOT THIS WAY AND WE QUERIED GND; WHO GAVE US NEW INSTRUCTIONS BUT DID NOT MENTION THE XING OF THE RWY. WE TAXIED ON AND TOOK OFF UNEVENTFULLY. UPON REACHING DEST; I CALLED ORD TWR AND ASKED IF I HAD CAUSED ANY HEADACHES. INDEED; I HAD. IT WAS MY RELIANCE ON THE FO; WHO WAS BRAND NEW WITH ONLY 3 TRIPS INTO ORD; MY MISHEARING OF THE CLRNC AND MY BELIEF THAT THAT WAS WHERE EVERYONE ELSE WENT AND WHERE WE USUALLY GO THAT CAUSED MY XING. I SHOULD; AND WILL BE FROM NOW ON; MUCH MORE VIGILANT IN BOTH LISTENING TO AND UNDERSTANDING CLRNCS; AND PAYING MORE ATTN TO WHERE I AM INSTRUCTED TO GO.