AN S80 FLC TURNS ONTO THE WRONG RWY DURING A NIGHT TAXI OP IN THE RAIN AT CYYZ; ON.

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LANDED RWY 23 LATE NIGHT IN RAIN. TURNED OFF ACTIVE AND WAS INSTRUCTED TO TAXI TO THE APRON VIA TXWY H TO RWY 15L ONTO THAT RWY; THEN TURN OFF AT TXWY R. I THOUGHT WE HAD CLRED RWY 23 PRIOR TO THE RWY 23/15R INTXN. COMBINATION OF THIS; THE LATE HR; THE VERY DARK TERRAIN AND THE RAIN CAUSED ME TO ACCIDENTALLY TURN ONTO RWY 15R WHEN I SAW THE DIMLY LIT RED RWY SIGN. GND SOUNDED SOMEWHAT AMUSED AS HE INFORMED US OF OUR MISTAKE AND WE IMMEDIATELY CLRED AT THE VERY NEXT TXWY; RETURNED TO TXWY HOTEL AND CONTINUED OUR TAXI TO THE GATE WITHOUT FURTHER INCIDENT. NOTE: ONLY RWY 24L AND RWY 23 WERE THE ACTIVE RWYS THIS EVENING; BUT I STILL CANNOT BELIEVE I TURNED ONTO THE WRONG RWY. I SAW THE RWY SIGN BUT DID NOT RECOGNIZE THE 'R' WHEN I SHOULD HAVE BEEN LOOKING AT AN 'L.' I HAD STUDIED THE ARPT LAYOUT PRIOR TO THE DSCNT AND APCH AND HAD MENTALLY MAPPED OUT THE TAXI RTE I WAS EXPECTING. UNFORTUNATELY; I DID NOT COUNT ON GND INSTRUCTING ME TO USE A RWY IN THE TAXI SEQUENCE. I WILL KNOW BETTER NEXT TIME. IT WOULD ALSO HELP IF WE COULD GET THE SIGNS A LITTLE BRIGHTER IN THIS PART OF THE ARPT.