ON GS INTERCEPT AT 2000 MSL FLT ON APCH TO HPN; FLC OF MLG HAD NMAC WITH SMA ON XING HDG.

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WE WERE VECTORED ALMOST TO THE N SHORE OF LONG ISLAND AT 2000 BEFORE BEING TURNED TO THE NW AND CLRED FOR THE APCH. AS WE APCHED THE SHORE LINE (AND THE GS) WE WERE TOLD TO SLOW AS MUCH AS POSSIBLE FOR A LIGHT ACFT ON FINAL. BOTH THE FO AND I WERE ALSO VISUALLY TRACKING NUMEROUS OTHER TARGETS. WE BOTH SAW A FAIRLY HIGH SPD TARGET TO OUR L FRONT AT WHAT WE ESTIMATED 300-500 BELOW US. THIS ACFT PASSED BELOW US JUST AS WE CAPTURED THE GS. WE QUESTIONED THE CTLR ABOUT THIS ACFT (WHICH HE DID NOT ALERT US TO) AND HE COMMENTED THAT IT WAS INCREDIBLE BUT LEGAL. THE INCONCEIVABLE FACTOR IS THAT THIS ACFT PROBABLY STAYED AT 1500 (OR SO) FROM THE PREVIOUS 1500 TCA RESTRICTION BUT HE COULD HAVE CLBED TO 3000 AND THROUGH THE GS ALT AT ANY TIME. IT MAKES NO SENSE AND IS DANGEROUS TO HAVE THE FLOOR OF THE TCA AT 3000 OVER LONG ISLAND SOUND ON THE FINAL APCH COURSE TO RWY 34 AT HPN. BASED ON THIS AND OTHER FLTS INTO HPN; I RECOMMEND STRONGLY THAT THE FLOOR OF THE TCA SHOULD BE 1500 IN THIS AREA AND THE TCA SHOULD EXTEND FURTHER N OF HPN.