B747 CREW MAY HAVE INADVERTENTLY CROSSED RWY 28L AT SFO BECAUSE OF SIMILAR CALL SIGN WITH ANOTHER ACFT ALSO XING RWY 28L.

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AS WE APCHED THE HOLD SHORT FOR RWY 28L AT SFO; THINGS WERE GOING SMOOTHLY -- ON TIME; NICE WX; ETC. THE CREW ON OUR FLT THAT DAY; ACR X; CONSISTED OF 4 INDIVIDUALS INSTEAD OF 3 BECAUSE WE HAD AN LCA GIVING A CHK TO THE SO; THUS THERE WERE 4 PAIRS OF EARS LISTENING TO TWR. WE ALL NOTED HOW BUSY THE GENTLEMAN IN THE TWR WAS; THE COMS WERE REALLY FLYING. AS A MATTER OF FACT; AFTER REACHING THE HOLD SHORT OF RWY 28L AND WAITING FOR APPROX 10-15 MINS WITHOUT COMS FROM TWR; WE ALL BEGAN TO WONDER IF HE HAD FORGOTTEN ABOUT US AND DEBATED GIVING HIM A CALL; BUT DECIDED AGAINST IT BECAUSE HE WAS VERY BUSY. OTHER NOTEWORTHY INFO: SFO WAS LNDG ON RWYS 28L&R AND DEPARTING RWY 1L; RWY 28L&R (MOST NARROW BODIES ON RWY 1L; IN FACT DURING OUR TIME AT HOLD SHORT NO ONE DEPARTED ON RWY 28L OR RWY 28R). IT WAS ALSO DURING THIS TIME THAT I NOTED AN ACR XYZ ALSO ON TWR FREQ; BUT I NEGLECTED TO LISTEN CAREFULLY ENOUGH TO FIND OUT THEIR POS. OVER THE NEXT 5 MINS OR SO; I HEARD THIS CALL SIGN 'ACR XYZ' SEVERAL MORE TIMES; BUT CAUGHT NO DETAILS. WHAT OCCURRED NEXT WAS FAST AND WHAT I HEARD WAS 'ACR YZ WITHOUT DELAY CROSS RWY 28L AND HOLD SHORT RWY 28R.' I REPEATED BACK THE INSTRUCTIONS AND WE EXECUTED THEM. THE NEXT THING WE HEARD FROM TWR WAS; 'ACR YZ I DON'T KNOW WHY YOU CROSSED RWY 28L; BUT HOLD SHORT OF RWY 28R.' I REPEATED BACK HIS INSTRUCTIONS AND QUICKLY TOLD HIM THAT WAS WHAT I UNDERSTOOD AND HAD REPEATED THEM BACK. HE DID NOT REPLY IMMEDIATELY. ABOUT 3 MINS LATER; TWR CAME BACK AND SAID; 'ACR YZ; SORRY ABOUT THAT; POS AND HOLD RWY 28R' AFTER WHICH WE TOOK OFF UNEVENTFULLY AND AFTER REACHING CRUISE BEGAN TO DISCUSS WHAT HAD HAPPENED. AS IT TURNED OUT; THE CAPT WAS BUSY AT THE TIME THE TWR CALLED 'US' THE FIRST TIME AND DID NOT HEAR WHAT TWR SAID; AND SINCE I WAS SURE WHAT I HEARD AND IT INCLUDED 'WITHOUT DELAY' I RELAYED TO THE CAPT THAT WE NEEDED TO CROSS RWY 28L IMMEDIATELY. WE CLRED OURSELVES OUT TO THE R AND SAW AN ACFT ON APCH TO RWY 28L AT WHAT LOOKED LIKE 3-4 MI; WHICH DID NOT OVERLY CONCERN ANYONE. AFTER XING RWY 28L AND GETTING REBUKED BY TWR; BOTH THE SO AND LCA SAID THEY HEARD THE SAME THING I DID. IT IS PROBABLY PRETTY OBVIOUS; AS IT IS TO MYSELF LOOKING BACK; THAT I SHOULD HAVE PAID CLOSER ATTN TO THE POS OF AN ACFT WITH SUCH A SIMILAR CALL SIGN. AFTER DISCUSSING IT AT LENGTH; WHAT WE THINK HAPPENED WAS THAT ACR XYA LANDED ON RWY 28R AND WAS WAITING FOR LNDG TFC TO CROSS RWY 28L AND THEN PROCEED TO THEIR GATE. WHEN TWR ISSUED THE CLRNC TO 'CROSS RWY 28L WITHOUT DELAY' IT IS POSSIBLE THAT BOTH ACR YZ AND ACR XYZ RESPONDED AT THE SAME TIME. WHAT EXACTLY TWR SAID IN THE ORIGINAL CLRNC; I AM STILL NOT SURE.