A B727-200 WAS GIVEN A DSCNT THEN CLB DURING WHICH THEY RECEIVED A TCASII RA ON DSCNT INTO IAD.

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FLT OPERATED SCHEDULED SVC FROM BOS-IAD. A B727-200 WAS GIVEN INSTRUCTIONS FROM ZNY AT APPROX XA00 TO DSND AND CROSS ROBRT INTXN AT 10000 FT AND 250 KIAS. THE ACFT BEGAN ITS DSCNT TO MAKE THE XING RESTR. WHILE DSNDING THROUGH FL185; THE ZNY CTLR ADVISED CREW TO STOP DSCNT AT FL190. WE ADVISED THE ZNY CTLR THAT WE WERE CURRENTLY DSNDING THROUGH FL185. THE CTLR ADVISED THE CREW TO CLB BACK TO FL190; AT WHICH TIME THE DSCNT TREND (APPROX 2500 FPM) WAS REVERSED; AND A CLB STARTED TO FL190. THIS REVERSAL FROM DSCNT TO CLB ACTIVATED THE TCASII 'TFC ALERT' FOLLOWED VERY QUICKLY BY AN RA REQUESTING A DSCNT BTWN 1500-2000 FPM. THE ACFT IMMEDIATELY RESPONDED TO THE TCASII DSCNT ALERT AND ADVISED ZNY OF OUR ACTIONS. THE 'BOGIE' ACFT PASSED WITHIN 5 MI AT AN ALT OF 600 FT ABOVE OUR FLT; ACCORDING TO THE TCASII DISPLAY. THE CAPT ASKED THE CTLR FOR HIS NAME; AFTER WHICH THE INITIALS WERE GIVEN FOLLOWED BY A PHONE NUMBER. FLT CONTINUED TO ITS DEST WITHOUT INCIDENT OR INJURY TO PAX. I BELIEVE AN OPERROR BY ZNY NEARLY RESULTED IN AN NMAC.