B757 CROSSED RWY IN FRONT OF B737 TKOF. LTSS. RWY INCURSION. SYS ERROR.

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THE EVENT DESCRIBED BELOW OCCURRED APPROX 1 MIN AFTER I WAS RELIEVED FROM THE LCL CTL POS. A B757 WAS HOLDING SHORT OF RWY 28L ON TXWY E; AN MD80 HAD LANDED RWY 28L AND WAS ROLLING OUT AS LCL CTL TAXIED A B737 INTO POS AND HOLD ON RWY 28L. LCL CTL USED THE CALL SIGN OF THE B757; BUT MEANT TO INSTRUCT THE MD80 TO EXIT THE RWY; HOLD SHORT OF THE OUTER; CONTACT GND. LCL CTL CLRED THE B737 FOR TKOF. BOTH THE MD80 AND THE B757 ACKNOWLEDGED AND THE B757 CROSSED RWY 28L AS THE B737 BEGAN TKOF ROLL. THE RADAR COORDINATOR POS WAS NOT STAFFED. WORKLOAD WAS MODERATE-HVY FOR LCL CTL/RADAR COORDINATOR COMBINED. THE LCL CTLR HAD TO WRITE DOWN ALL ARRS; CALL OFF DEPS; TO DEPT CTLR AND ENSURE ACCURACY OF ALL DEP STRIPS IN ADDITION TO HIS LCL CTL DUTIES. HAD THE RADAR COORDINATOR POS BEEN STAFFED; THE INCIDENT VERY LIKELY COULD HAVE BEEN AVOIDED. THE RADAR COORDINATOR WRITES DOWN ARR CALL SIGNS; KEEPS TRACK OF THE ARRS HOLDING BTWN THE RWYS; LINES OUT CALL SIGNS AS ACFT ARE SWITCHED TO GND CTL AND MONITORS LCL CTL FREQ TO ASSIST AS NECESSARY. IN ADDITION THE PLT OF THE B757 CROSSED THE ACTIVE RWY ON THE ASSUMPTION THAT HE HAD RECEIVED CLRNC. THE WORDS 'TAXI ACROSS' WERE NEVER USED BY LCL CTL -- THUS THE PLT SHOULD HAVE QUESTIONED THE INSTRUCTIONS. SUPPLEMENTAL INFO FROM ACN 304653: AFTER LNDG SFO RWY 28R AND HOLDING SHORT ON TXWY E -- TWR CTLR CLRED US TO CROSS RWY 28L ON THE HIGH SPD TO JOIN THE OUTER TXWY AND SWITCH TO GND. AFTER THE CAPT APPLIED TAXI PWR TO CROSS RWY 28L; TWR PROCEEDED TO CLR A JET FOR TKOF ON RWY 28L ALSO. BY THIS TIME WE WERE APPROX 1/2 ACROSS RWY 28L AND CONTINUED OUR TAXI. WE HAD CLRED THE RWY PRIOR TO THE DEPARTING FLT XING OUR PATH (IE; NO CONFLICT). WE COMMENTED AFTER SWITCHING GND HOW UNUSUAL IT WAS TO HAVE A 'CLRNC TO CROSS' FOLLOWED SHORTLY AFTER WITH A 'CLRED FOR TKOF' COMMAND. WE THOUGHT THAT IT MIGHT BE LCL SOP BECAUSE OF THE DISTANCES INVOLVED. THE DEPARTING JET COULD HAVE HELD IN POS AFTER HEARING OUR CLRNC TO CROSS; AND QUESTION HIS 'CLRED FOR TKOF' CLRNC WITH US ON THE RWY -- WE WERE ON THE SAME TWR FREQ.