NMAC BTWN ACR LGT AND SMT Y ON APCH AT DEN; TCASII RA MAY HAVE PREVENTED COLLISION.

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WHILE FLYING A VISUAL APCH TO RWY 26R AT DENVER/STAPLETON; TCASII PROVIDED A RA TO CLR UNSEEN TFC Y. RA WAS COMPLIED WITH AND BECAUSE OF DISTANCE TO RWY; A GAR WAS EXECUTED. AN UNEVENTFUL VISUAL APCH TO RWY 17R WAS FLOWN AND LNDG COMPLETED. BEING VECTORED BY APCH HDG 265 DEG/8000 FT/ 7 1/2 MI OUT FROM RWY 26R; BUT DISPLACED ABOUT 1 MI TO R OF RWY CTRLINE 26R DUE TO NUMEROUS TFC TARGETS IN AREA. ASSIGNED HDG OF 265 DEG ALLOWED US TO PARALLEL THE LNDG RWY. APCH CTL WAS OVERLOADED AND KEPT US HIGH AND OFFSET WITH NO LNDG CLRNC. WHEN QUESTIONED ABOUT INTENTIONS A NEW VOICE (NEW CTLR) TOOK OVER AND OVERRODE OTHER CTLR CLRING US FOR VISUAL TO 26R. AS WE SIDE STEPPED TO L TO ACQUIRE CTRLINE AND DSNDED TO REGAIN DSCNT; AT APPROX 1500 FT AGL RECEIVED TCASII 'TFC; TFC' WITH TARGET BELOW TCASII ACFT SYMBOL. WITHIN 2-3 SECONDS RA 'MONITOR VERT SPD' WITH CLB OF 150 FPM OCCURRED. WE COMPLIED WITH RA AND CLBED TO 7200 FT MSL AND TCASII ADVISED 'CLR OF CONFLICT.' WE ADVISED TWR OF TCASII WARNING. WE NEVER SAW TFC BELOW US AND WERE NOT ADVISED BY ATC. NOTIFIED TWR OF RA AND DUE TO CLOSENESS TO RWY; WENT AROUND. ON GND; CAPT TALKED TO TWR SUPVR WHO ADVISED US THAT SMT Y WAS ON FINAL TO RWY 25 AND IN CONTACT WITH TWR AND WAS DIRECTLY BELOW US; BUT SINCE WE WERE ON APCH CTL; WE HAD NO KNOWLEDGE OF HIM. WX WAS VFR BUT ATC WAS FAR TOO OVERLOADED AND PRESSED TO HANDLE MANY ACFT TO THOSE 3 RWYS. I AM A FIRM BELIEVER IN TCASII -- THE SYS WORKS. SUPPLEMENTAL INFO FROM ACN 203144. APCH CTL WAS TRYING TO HARD TO FIT TOO MANY ACFT INTO 3 VERY CLOSELY SPACED PARALLEL RWYS (25/26R/L). I CONTACTED TWR MGR BY PHONE AND HE ADVISED THEN THAT THERE WAS SMT Y ON TWR FREQ. DIRECTLY BELOW US LINED UP ON RWY 25 AND CLRED TO LAND. WE WERE NOT ADVISED OF THIS TFC BY APCH; COULD NOT HEAR HIM BEING 'WORKED' AS HE WAS ON TWR FREQ AND WE ON APCH; AND COULD NOT VISUALLY SEE HIM DUE TO HIS POS RELATIVE OUR ACFT (DIRECTLY BELOW). I STRONGLY BELIEVE; AS DOES MY CREW THAT WE WOULD HAVE DSNDED ON TOP OF THIS ACFT HAD IT NOT BEEN FOR TCASII. TWR MGR THOUGHT IT WAS NO BIG DEAL AS NO CLRNCS WERE VIOLATED DURING RA AND AVOIDANCE MANEUVER. WE THOUGHT IT WAS A BIG DEAL.