ACFT UNDER TOW OCCUPIES RWY AT SFO; CA; WHILE ANOTHER ACR ACFT LANDS ON THE SAME RWY.

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OUR ACFT WAS NOT INVOLVED. WE OBSERVED THE INCIDENT FROM THE INTXN OF TXWYS B AND D. 5 MINS PRIOR TO INCURSION; I HEARD GND CTL (AT SFO) CLR A 'Y' ACFT UNDER TOW TO 'CROSS RWY 28L; HOLD SHORT OF RWY 28R.' THE MECH FUMBLED THE READBACK THE FIRST TIME; BUT GOT IT CORRECT THE SECOND TIME. BY THE TIME WE GOT TO THE ABOVE-MENTIONED INTXN; THE ACFT UNDER TOW WAS JUST BEGINNING TO CROSS RWY 28L. I DON'T KNOW WHY HE DELAYED SO LONG. WE LOOKED UP TO SEE A PLANE (ANOTHER ACR Z -- DON'T KNOW WHO IT WAS BECAUSE WE WERE NOT LISTENING TO THE LCL POS) ON SHORT FINAL. WE COULD NOT TELL IF HE WAS LINED UP FOR RWY 28R OR RWY 28L (NIGHTTIME). THE ACFT UNDER TOW STOPPED IN THE MIDDLE OF RWY 28L (XING AT TXWY E) JUST AS THE OTHER ACFT TOUCHED DOWN ON RWY 28L. THE LNDG ACFT Z STOPPED SHORT; AND THERE WAS NO COLLISION. MY OPINION: THE LCL CTLR GAVE PERMISSION TO THE GND CTLR TO LET THE ACFT UNDER TOW CROSS RWY 28L; BUT HE TOOK SO LONG TO DO IT THAT THE LCL CTLR FORGOT ABOUT IT AND CLRED SOMEONE ELSE TO LAND. THERE WERE NO COMBINED ATC POS; AND THE WX WAS CLR VFR. I DON'T KNOW WHO IS RESPONSIBLE HERE; BUT THE INCIDENT NEEDS TO BE DOCUMENTED. WE'VE GOT TO STOP THESE RWY INCURSIONS.