A TAXIING B737-200 FLC CROSSES AND STOPS OVER THE HOLD SHORT LINE FOR RWY 28C AT PIT. FO'S EYES WERE 'INSIDE' THE COCKPIT WORKING OUT A RERTE DURING THIS NIGHT FLT.

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AFTER PUSHBACK FROM GATE AND CONTACT WITH ATC GND CTL; WE WERE INFORMED THAT ZOB WAS NOT ACCEPTING DEPS FOR OUR DIRECTION OF FLT (PRESUMABLY DUE TO CONVECTIVE WX). A LENGTHY GND HOLD ENSUED WHILE AWAITING A RERTED CLRNC. WE WERE ALREADY WELL BEHIND SCHEDULE DUE TO A MECHANICAL DELAY ON THE PREVIOUS LEG; FOLLOWED BY ARR HOLDING DUE TO TSTMS AT PIT. WE FINALLY RECEIVED OUR RERTED CLRNC AND WERE TAXIING TO RWY 28C VIA TXWY V AND TXWY E. I WAS PULLING OUT ENRTE CHARTS TO LOOK UP AND VERIFY OUR NEW RTE OF FLT (COMPLETE RERTE). I WAS THEREFORE UNABLE TO BACK UP THE CAPT AS HE TAXIED THE ACFT. MY EYES WERE 100% INSIDE THE ACFT; SPLIT COCKPIT DUTIES. I FELT SAFE DOING THIS AS WE WERE CLR OF THE CONGESTED AREA AND THE CAPT MUST HAVE TAXIED THIS RTE THOUSANDS OF TIMES. NEVERTHELESS; THE CAPT FAILED TO SEE THE RWY SIGNS AND RATHER THAN MAKE A L TURN ON TXWY E; THOUGHT RWY 28C WAS TXWY E. WE WERE STOPPED FROM ACTUALLY ENTERING THE RWY BY AN ALERT GND CTLR. THE CAPT IS A VERY SHARP PLT WHO IS ALWAYS ON TOP OF THINGS. HE TOLD ME HE WAS EXPECTING TO SEE THE SIGN FOR RWY 28C; BUT NEVER SAW IT SO CONTINUED STRAIGHT AHEAD. MY HEAD WAS DOWN SO I CANNOT VERIFY; BUT I WONDER IF THE SIGN WAS EVEN ILLUMINATED. IT WAS A DARK AND WET NIGHT; SO VISIBILITY WASN'T THE BEST; BUT IT WAS NOT ACTUALLY RAINING AT THE TIME OF THE INCIDENT. I'M SURE FATIGUE PLAYED A PART AS WELL. IN THE FUTURE I WILL ALWAYS SUSPEND OTHER COCKPIT DUTIES TO MONITOR THE CAPT AS HE APCHS TURN POINTS WHICH IF MISSED COULD LEAD TO A RWY INCURSION. FACTORS: FATIGUE POSSIBLY; REDUCED VISIBILITY AND SPLIT COCKPIT DUTIES FOR SURE; AND POSSIBLY A SIGN THAT WAS NOT ILLUMINATED. SUPPLEMENTAL INFO FROM ACN 402894: I WAS CLRED TO TAXI VIA TXWY V AND THEN L ON TXWY E; BUT FAILED TO SEE THE RWY MARKING SIGNS. THE RWY AND TXWY SIGN AT PIT ARE THE NEW DESIGN LIGHTED SIGNS BUT WERE NOT OBVIOUS ENOUGH TO COMMAND MY ATTN. UPON RETURNING TO THE SAME LOCATION THE NEXT DAY DURING DAYLIGHT; I NOTICED THAT THE TXWY AT THAT INTXN IS VERY WIDE AND THE RWY SIGNS ARE FARTHER THAN I EXPECTED FROM THE TXWY CTRLINE. I WAS FOCUSING ON THE TXWY AT THE TIME OF THE INCIDENT DUE TO THE EFFECT OF 'TUNNEL VISION' DURING A DISTR OR BUSY EVENT; AND THAT PROBABLY CONTRIBUTED TO MISSING THE SIGNS. PIT HAS YELLOW LIGHTS ON THE TXWY CTRLINES AT MANY LOCATIONS ON THE ARPT AT POTENTIALLY HAZARDOUS INTXNS; BUT NOT AT THIS ONE THAT NIGHT. I FEEL THIS LOCATION ON CTRLINE IS BETTER FOR WARNING LIGHTS BECAUSE WHEN VISIBILITY IS LIMITED OR RESTR; THAT IS THE AREA THAT GETS THE MOST ATTN; RIGHT IN FRONT OF THE ACFT WHERE IT IS MOVING; NOT 100 FT OR SO OFF TO THE SIDE.