FLC OF ACR LTT ACFT WERE ALARMED BY A NEAR TAXI MISS BY AN SMT ACFT.

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INSTRUCTED BY BOS TWR TO 'POS AND HOLD ON THE NUMBERS' RWY 4L. HAD JUST BEGUN TO MOVE ACFT FROM A STANDING START ON S TAXIWAY INTO POS WHEN MY FO AND I WERE SURPRISED BY A TWIN ACFT SMT; ALSO TAXIING; OVERTAKING US FROM BEHIND ON OUR R; MOVING FAST. THIS ACFT APPARENTLY HAD COME UP BEHIND US ON INTERSECTING TAXIWAY E AND WAS TRANSITING ACROSS RWY 4L ON INSTRUCTIONS FROM GND CTL (MY PRESUMPTION AS NEITHER MY FO NOR I RECALL HEARING ANY INSTRUCTIONS TO OTHER ACFT ON TWR FREQ REGARDING OUR RWY. ESTIMATE WING TIP PASSED WITHIN 3-4 FT OF OUR ACFT. NO NOTIFICATION WAS GIVEN TO US REGARDING THIS OTHER ACFT THAT HAD BEEN GIVEN INSTRUCTIONS TO TAXI IN EXTREMELY CLOSE PROX TO OUR ACFT. THE ACFT WAS NOT YIELDING AND IN FACT SEEMED TO BE ACCELERATING; DESPITE THE FACT THAT HE WAS THE OVERTAKING ACFT. AFTER THE CONFLICT OCCURRED IT BECAME QUITE CLR WHY THE CTLR INSTRUCTED US TO 'POS AND HOLD ON THE NUMBERS.' THIS WAS TO ALLOW ROOM FOR THE OTHER ACFT TO CROSS THE RWY BTWN OUR ACFT AND THE RWY THRESHOLD. THE CTLR SHOULD HAVE TOLD US ABOUT THE OTHER ACFT.