FLC AT FOREIGN LSZH ARPT INADVERTENTLY CROSS RWY WITHOUT PERMISSION AFTER BEING INSTRUCTED BY RAMP CTL TO FOLLOW ANOTHER ACR ACFT WHEN TAXIING OUT.

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WHILE TAXIING TO RWY 16 AT ZURICH INTL ARPT; GND CTL INFORMED THE CREW OF ACR Y FLT ENRTE TO EWR THAT WE HAD CROSSED RWY 28 WITHOUT PERMISSION. OUR TAXI CLRNC FROM APRON CTL WAS 'TAXI TO RWY 16 VIA TXWY E TO HOLD SHORT OF RWY 28.' WE ACKNOWLEDGED AND READ BACK THE CLRNC. WHILE APCHING RWY 28; APRON CTL TOLD US TO FOLLOW AN ACR Z B737 ACROSS RWY 28 AND MONITOR GND CTL ON FREQ 128.1 (APRON CTL WAS ON FREQ 121.75). WE FOLLOWED THE ACR Z B737 ACROSS RWY 28 AND MONITORED GND WHO INFORMED US THAT WE WERE NOT CLRED TO CROSS RWY 28 BY APRON CTL AND THAT A RPT WOULD BE FILED. I WAS FLYING AS FO FROM THE L SEAT TAXIING THE ACFT (B767-200). I WAS BEING GIVEN A CAPT'S LINE CHK BY THE CAPT CHK AIRMAN. OCCUPYING THE JUMP SEAT WAS AN ASSIGNED IRO WHO WAS THE THIRD CREW MEMBER. WE ALL AGREED THAT OUR CLRNC WAS TO FOLLOW ACR Z ACROSS RWY 28. I CHKED THE L SIDE VISUALLY TO THE APCH END OF RWY 10 AND THE CAPT IN THE R SEAT CHKED THE APCH TO RWY 28 AND SAW ANOTHER ACFT WAITING IN POS ON RWY 28. THIS WAS NOT UNUSUAL; SO WE CROSSED RWY 28. WHEN INFORMED THAT WE CROSSED RWY 28 WITHOUT PERMISSION; THE CAPT EXPLAINED THAT WE THOUGHT WE HAD CLRNC TO CROSS AND APOLOGIZED FOR THE MISUNDERSTANDING AND REQUESTED THE TAPES BE PULLED FOR CONFIRMATION. WE CONTINUED TAXIING AND SUBSEQUENTLY TOOK OFF AND FLEW TO EWR WITHOUT FURTHER INCIDENT.