WRONG RWY TKOF ATTEMPTED ON A CLOSED RWY BY A B737-200 FLT CREW AT ONT; CA.

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WHILE TAXIING OUT FOR TKOF EBOUND ON TXWY N; THE TWR CLRED US FOR TKOF PRIOR TO REACHING TXWY V. (RWY 26L WAS IN USE.) THE CAPT THEN CALLED FOR BELOW THE LINE; AND AS THE CHKLIST WAS BEING PERFORMED; WE TAXIED INTO POS ON RWY 26R. UPON COMPLETION OF THE CHKLIST; THE THRUST LEVERS WERE ADVANCED; AND THE CAPT REQUESTED TKOF PWR TO BE SET. AS WE STARTED ROLLING; THE TWR CALLED TO CANCEL OUR TKOF AND ADVISED US THAT WE WERE ON A CLOSED RWY. THE CAPT CLOSED THE THRUST LEVERS AND BRAKED; ALLOWING US TO TURN OFF ONTO THE NEXT AVAILABLE TXWY. (THE AIRSPD NEVER REACHED 60 KTS.) THE TWR ADVISED THE CAPT THAT NO ACTIONS WOULD BE TAKEN ON HIS PART. WE THEN TAXIED BACK FOR A NORMAL TKOF ON RWY 26L. THERE WAS A LIGHTED X ON A STAND ON THE APCH END OF RWY 26R. ACFT LNDG WOULD SEE IT QUICKLY. HOWEVER; UNLESS YOU WERE LOOKING TO THE L (E) YOU WOULD NOT NOTICE THIS RWY BEING CLOSED. ANOTHER FACTOR CONTRIBUTING TO THIS EVENT IS THAT RWY 26R IS THE NORMAL RWY FOR TKOF WHEN WESTERLY DEPS ARE IN USE. I FEEL THAT THE 'X' WOULD HAVE BETTER VISIBILITY AND EFFECTIVENESS IF IT WERE PLACED ON THE RWY SO THAT IT WOULD BE VISIBLE TO BOTH DEPARTING AS WELL AS LNDG ACFT. SUPPLEMENTAL 579515: THE TWR CLRED US FOR TKOF. WE ALSO HEARD AN ACFT BEING CLRED TO LAND; SO WE FELT WE NEEDED TO EXPEDITE OUR TKOF. MY ATTN WAS DIVERTED BTWN THE LNDG ACFT AND PERFORMING THE 'BELOW THE LINE ITEMS.' I INADVERTENTLY TURNED ON RWY 26R AND STARTED OUR TKOF ROLL. TWR NOTICED OUR MISTAKE AND NOTIFIED US RWY 26R WAS CLOSED. WE TURNED OFF THE NEXT TXWY AND RETURNED TO RWY 26L FOR A NORMAL TKOF AND DEP. TWR SAID THERE WAS NO PROB AND NOTHING WAS GOING TO BE RPTED. CONTRIBUTING FACTORS: IT WAS DUSK; AND VISIBILITY WAS REDUCED. RECEIVING A TKOF CLRNC ON THE TXWY WELL BEFORE THE HOLD SHORT AREA OF BOTH RWYS. MY PAYING TOO MUCH ATTN TO THE LNDG ACFT AND NOT WANTING TO DELAY OUR TKOF ROLL.