B737 FLT CREW TAXIES ACROSS HOLD SHORT LINE AT ONT.

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UPON LNDG ROLLOUT ON RWY 26R IN ONT; CA; WE WERE GIVEN TAXI INSTRUCTIONS TO TURN L ON TXWY K; L ON TXWY M; AND HOLD SHORT RWY 26R AT TXWY P. ON APCH; I WAS EXPECTING THE NORMAL R TURN AND TAXI IN VIA TXWY P; TXWY N; AND TXWY U. WITH NUMEROUS TXWYS CLOSED AND A SLIGHTLY LONGER LNDG ROLL THAN ANTICIPATED; DUE TO A SLIGHT TAILWIND; WE WENT PAST TXWY P AND REQUIRED THE L TURN RTE. AS WE EXITED AT TXWY K; I CALLED FOR 'FLAPS UP CLEAN UP.' I WAS TAXIING SLOWLY AND IN THE BACK OF MY MIND I WAS THINKING THAT THIS WAS WHERE WE HAD A CREW END UP ON RWY 26L BY MISTAKE. TXWY M IS VERY CLOSE TO RWY 26L; JUST 1 ACFT LENGTH; AND AS I WAS STRAIGHTENING THE ACFT FROM THE L TURN ONTO TXWY K; THE NOSEWHEEL WAS ALREADY ON TXWY M. I LOOKED AND DIDN'T SEE ANY TXWY SIGNS. I SAW BLACKTOP ASPHALT RUNNING PARALLEL AND CLOSE TO RWY 26R. IT DIDN'T LOOK LIKE A TXWY TO ME. IT LOOKED LIKE A SURFACE VEHICLE ROAD. THE REST OF THE RWYS AND TXWYS IN ONT ARE CONCRETE. TXWY M IS THE ONLY THING ON THE SURFACE THAT IS BLACKTOP ASPHALT; IT HAS HUMPS IN IT AND IT IS NOT PERFECTLY STRAIGHT LIKE THE OTHER TXWYS AND RWYS. AS WE PASSED TXWY M; THE FO AND I WERE DOING OUR 'FLAPS UP; CLEAN UP' FLOWS WHEN THE TWR CALLED AND TOLD US THAT WE HAD PASSED TXWY M. I REALIZED THAT I HAD BEEN LOOKING AT RWY 26L AND THINKING THAT IT WAS TXWY M. I SAW THE HOLD SHORT LINES FOR RWY 26L APCHING AND HAD TO BRAKE FAIRLY HARD TO AVOID GOING OUT ONTO RWY 26L. HAD THE TWR CTLR NOT CAUGHT THE ERROR I AM SURE THAT I WOULD HAVE TAXIED THE ACFT ONTO RWY 26L. THERE WAS NO TFC ON FINAL FOR RWY 26L; SO THE TWR CTLR CLRED US TO BACK-TAXI ON RWY 26L AND TAKE TXWY P ACROSS RWY 26R THEN TO THE GATE. I KNEW THE AREA WAS TRICKY; I HAD LOOKED AT MY 10-9 PAGE AND HAD IT OUT; AND I STILL MADE THE MISTAKE. CONTRIBUTING FACTORS: THE CLRNC WAS NOT WHAT I HAD EXPECTED. I HAD NEVER TAKEN THIS TAXI RTE BEFORE. THE FO HAD NEVER TAKEN THIS TAXI RTE BEFORE. WE WERE BOTH DOING OUR 'FLAPS UP; CLEAN UP' FLOWS. THE TXWY IS UNUSUALLY CLOSE TO RWY 26L. THE TXWY IS BLACKTOP; WHILE THE REST OF THE ARPT IS CONCRETE; GIVING IT THE APPEARANCE OF A ROAD. RWYS 26R&L ARE CLOSE TOGETHER. THIS AREA IS NOT WELL MARKED; IT MAY NOT BE MARKED AT ALL. I DON'T THINK TXWY M IS STRIPED.