AFTER LNDG CYYZ; AN ACR PLT CAPT STARTS TO CROSS HOLD LINE OF PARALLEL RWY BUT IS STOPPED BY THE FO WITH NOSE OF ACFT SLIGHTLY OVER LINE.

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CAPT FLYING. PRIOR TO LNDG; STUDIED ARPT DIAGRAM CAREFULLY; DUE TO LNDG ON OTHER THAN USUAL RWY; INCLUDING POTENTIAL RTE FROM RWY TO GATE. SOMEWHAT FAMILIAR WITH ARPT (40-50 LNDGS OVER 20 YRS) ASSIGNED RWY 6R AS EXPECTED. FOR UNKNOWN REASON; WHEN I WAS DONE REVIEWING THE TAXI DIAGRAM; I WAS MENTALLY PICTURING A CTR PARALLEL TXWY BTWN RWY 6R AND RWY 6L. THAT TXWY DOES NOT ACTUALLY EXIST. LANDED RWY 6R NORMALLY; EXITED ON D3 WITH A CLRLY HEARD CLRNC TO HOLD SHORT OF RWY 6L. I WAS CONVINCED THAT THE FIRST INTXN WAS THE NON EXISTENT CTR TXWY; AND WAS FOCUSED ON HOLDING SHORT OF THE FOLLOWING INTXN. THE FO RECOGNIZED THE IMPENDING SIT; AND DECISIVELY AND ASSERTIVELY APPLIED BRAKES AND SAID 'HOLD SHORT.' I RECOGNIZED THE ERROR AT THAT POINT. THE ACFT STOPPED WITH THE NOSE JUST OVER THE HOLD SHORT LINE. NOTIFIED TWR IMMEDIATELY THAT WE HAD ROLLED SLIGHTLY PAST THE LINE; NO APPARENT CONFLICT; RECEIVED CLRNC TO CONTINUE. THIS WAS A POSITIVE CASE OF THE FO ACTIVELY MONITORING THE TAXI; WHICH IS; UNFORTUNATELY NOT ALWAYS THE CASE. HE HAS RECEIVED TRAINING FROM THE AIRLINE IN ASSERTIVENESS; AND HIS ASSERTIVENESS AND DECISIVENESS WERE EXACTLY APPROPRIATE TO THE SIT; AND CLRLY PREVENTED A FULL-BLOWN INCURSION ONTO THE ACTIVE RWY. ANOTHER POSITIVE FACTOR WAS THE AIRLINE'S POLICY OF NOT READING THE 'AFTER LNDG' CHKLIST UNTIL ALL ACTIVE RWYS HAVE BEEN CROSSED INBOUND. I CLRLY COULD HAVE AVOIDED THE SIT BY STATING THAT I WAS PLANNING ON HOLDING SHORT OF THE SECOND INTXN; WHICH WOULD HAVE ALLOWED THE FO TO CATCH MY ERROR OF PERCEPTION SOONER.