A320 FLT CREW WAS CLEARED TO TAXI ACROSS APCH END OF RWY 4L TO RWY 4R. TAXI SPEED WAS RELATIVELY SLOW AND FLT CREW DID NOT REALIZE ACFT WAS ON FINAL FOR RWY 4L. ATC RECOGNIZED POTENTIAL CONFLICT AND INFORMED FLT CREW TO EXPEDITE TAXI.

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I WAS THE FO ON AN AIRBUS A320 OPERATING FROM BOSTON; MA. OUR ORIGINAL TAXI CLRNC FROM THE GATE TO THE RWY WAS TO TAXI VIA TXWY A HOLD SHORT OF TXWY C. AT TXWY C WE RECEIVED CLRNC TO TAXI TO RWY 4R VIA TXWYS A; K3; B; HOLD SHORT OF THE TXWY B HOLD POINT. AFTER HOLDING FOR A SHORT PERIOD AT THE TXWY B HOLD POINT WE WERE CLRED TO RWY 4R VIA TXWY B; HOLD SHORT OF THE RWY 4R VISUAL HOLD POINT. WE TAXIED PAST THE TXWY B HOLD POINT TOWARDS RWY 4R AT A RELATIVELY SLOW PACE. I LOOKED TO THE R TO CLR FINAL AS WE APCHED THE EXTENDED CTRLINE OF RWY 4L AND TOLD THE CAPT CLR R. AS WE PROCEEDED ACROSS THE EXTENDED CTRLINE OF RWY 4L TWR TOLD US TO EXPEDITE OUR TAXI. I REPEATED THE INSTRUCTIONS TO EXPEDITE BACK TO THE TWR AND LOOKED TO OUR R. I WAS VERY SURPRISED TO SEE WHAT APPEARED TO BE A B717 ON SHORT FINAL FOR RWY 4L. AS I WAS DOING THIS THE CAPT ACCELERATED THE ACFT. AFTER THE ACFT PASSED OVER/BEHIND US I COULD SEE OUT THE L-HAND WINDOW THAT HE WAS GOING AROUND. I CAN RECALL THE IMAGE ON FINAL WHEN I TOLD THE CAPT IT WAS CLR R AS WE APPROACHED THE EXTENDED CTRLINE FOR RWY 4L. I COULD SEE LNDG LIGHTS ON FINAL FOR BOTH RWY 4L AND R. IT WAS DUSK; AND THE ACFT THAT IN RETROSPECT WAS THE ONE THAT APPROACHED OURS HAD ONLY ITS NOSE GEAR LIGHTS ON; THOUGH I BELIEVE THE B717 IS EQUIPPED WITH WINGTIP LIGHTS. I SUSPECT I INCORRECTLY INTERPED THIS TIGHT GROUP OF LIGHTS TO BE A DISTANT ACFT. THIS SITUATION COULD HAVE BEEN AVOIDED IN SEVERAL DIFFERENT WAYS. FIRST; I COULD'VE CLRED THE FINAL APCH COURSE BETTER. I SAW THE ACFT BUT DID NOT CORRECTLY PERCEIVE ITS DISTANCE. SECOND; WE COULD'VE TAXIED FASTER; WHICH WE CERTAINLY WOULD HAVE DONE IF (THIRD) THE TWR HAD INDICATED THERE WAS ANY URGENCY TO OUR TAXIING TO RWY 4R. IT IS INTERESTING TO NOTE THAT RAILROAD LOCOMOTIVES HAVE LIGHTS ON THEIR FRONT ARRANGED IN A TRIANGULAR PATTERN. THIS HELPS PEOPLE OBSERVING THE TRAIN APCH JUDGE DISTANCE AND RATE OF CLOSURE. I HAD NOT THOUGHT ABOUT THIS BEFORE; BUT WING MOUNTED LIGHTS ON AN ACFT PROVIDE THE SAME INFO. THE GREATER THE SEPARATION BTWN LIGHTS THE EASIER IT IS FOR AN OBSERVER TO JUDGE DISTANCE AND SPD.