A B747-400 TAXIING AT ORD WAS INVOLVED IN A NEAR COLLISION WITH AN RJ.

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WE HAD BEEN CLRED TO TAXI FROM GATE VIA TXWY A TO TXWY A9 AND TXWY T. I TAXIED FORWARD TOWARDS TXWY A AND ENDED UP STOPPING JUST SHY OF TXWY A DUE TO AN RJ WAITING FOR GUIDANCE IN TO A GATE AT THE S END OF THE C CONCOURSE. THE FO ASKED GND IF WE COULD TAXI OUT TXWY A6 TO GET AROUND THE RJ; BUT HIS XMISSION WAS BLOCKED. THE NEXT THING I SAW WAS ANOTHER RJ WITH ITS L WING GOING UNDER OUR NOSE. HE WAS MOVING VERY FAST AND HAD I NOT ALREADY STOPPED I AM QUITE SURE THERE WOULD HAVE BEEN A COLLISION. WHEN I HAD FIRST STARTED TAXIING OUT; THE FO HAD NOT SEEN ANYONE COMING AROUND THE R SIDE OF US BUT AS FAST AS THIS ACFT WAS MOVING; I AM NOT SURPRISED HE DIDN'T SEE IT AS IT PROBABLY HAD NOT BEEN IN HIS SIGHT FIELD. THE FO IMMEDIATELY NOTIFIED GND CTL OF THE NEAR MISS AND HE RESPONDED THAT THEY HAD SEEN IT IN THE TWR AND 'WOULD TAKE CARE OF IT.' I DO NOT HAVE THE FLT NUMBER OF THE RJ; BUT I HOPE THAT CAPT WILL THINK TWICE ABOUT ATTEMPTING TO OUT-RUN AN ACFT 10 TIMES HIS SIZE. I HAVE NOTICED BEFORE THAT THE INBOUND GND CTLR AND THE OUTBOUND GND CTLR DO NOT ALWAYS SEEM TO BE COMMUNICATING AS I HAVE HAD TO STOP FOR INBOUND ACFT MANY TIMES AFTER HAVING BEEN CLRED TO TAXI. THIS WAS A VERY NEAR MISS THAT COULD HAVE CAUSED SUBSTANTIAL ACFT DAMAGE TO BOTH ACFT.