CAPT ON L1011 AT SFO GIVEN TAXI INSTRUCTIONS ON TWO OCCASIONS TO TAXI ON OR ACROSS A RWY WHEN OTHER TFC WAS A FACTOR.

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WE WERE HOLDING SHORT OF RWY 28R IN SFO; WHEN WE WERE INSTRUCTED BY THE TWR OPERATOR (FEMALE) TO TAXI ONTO RWY 28R; PROCEED DOWN TO TXWY W; THEN R TURN ON TXWY C TO HOLD SHORT OF RWY 28R ON TXWY C. 'ACFT ON 2 1/2 MI FINAL.' THERE WAS A B747 ON FINAL FOR RWY 28R AT THE TIME. MY FO AND I CHECKED THE FINAL APCH PATH; SAW THE B747 AND COULD NOT BELIEVE THE CLRNC. I HELD POS AND LOOKED AT MY TXWY CHART TO SEE HOW FAR DOWN RWY 28R TXWY W WAS. THE FO QUERIED THE TWR CTLR AND ASKED HER 'YOU WANT US ON THE OPPOSITE SIDE OF THE RWY FACING THIS WAY; RIGHT?' THE TWR OPERATOR REPEATED THE PREVIOUS CLRNC; FOR US TO TAXI DOWN THE RWY; ETC; THEN ADDED ACFT Y? (FLT NUMBER); CHANGE YOUR RWY TO RWY 28L. AFTER THE B747 (NOW ON ABOUT A 2 MI FINAL) TURNED FOR RWY 28L AND ACKNOWLEDGED; 'WE FOLLOWED THE CLRNC.' AFTER THE FLT I CALLED SFO TWR AND SPOKE WITH THE SUPVR AND ASKED HIM TO LISTEN TO THE TWR TAPES AND CALL ME BACK. HE DID; AND SAID AT THE END OF THE FIRST CLRNC THE TWR CTLR HAD SAID 'TFC ON 2 1/2 MI FINAL MOVING L.' I INFORMED HIM 'MOVING L' IS NOT STANDARD TERMINOLOGY; AND; IN ADDITION; AT THE TIME WE RECEIVED OUR CLRNC TO TAXI DOWN THE RWY; THE ACR B747 WAS NOT 'MOVING L;' HE WAS LNDG ON RWY 28R! I INFORMED HIM I WOULD NOT ACCEPT A CLRNC THAT APPEARED UNSAFE; SO I HELD POS. PREVIOUSLY; ON SEP/XA/99; ABOUT XB:00 LEAVING RWY 1R RUNUP AREA; AFTER RUNNING UP AN ENG; IN SFO THE SAME CAUTION HAD PAID OFF. WE WERE CLRED BY THE TWR (A FEMALE VOICE; PERHAPS THE SAME ONE) TO 'CROSS RWY 1R; HOLD SHORT RWY 28R.' WE CROSSED RWY 1R (AFTER CHECKING FOR ACFT; AS IS NORMAL PROC) THEN STOPPED TO CONFIRM OUR FURTHER CLRNC. WE LOOKED FOR ACFT MOVING ON 1L AND AN ACFT WAS TAKING OFF! HAD WE CONTINUED AND PROCEEDED TO RWY 28R BEFORE HOLDING; WE WOULD PROBABLY HAVE BEEN IN ITS PATH.