B737 FLT CREW RESPONDED TO TCAS RA WHILE CRUISING AT FL350. ACFT IN HOLDING PATTERN HAD STRAYED FROM ITS ASSIGNED AIRSPACE.

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CRUISE AT FL350; CLR WITH WHITE CLOUD DECK ON HORIZON. GOT TFC ALERT; ACFT SAME ALT; RAPIDLY FOLLOWED BY RA 'DSND.' CAPT IMMEDIATELY BEGAN RESPONSE TO RA AND WELL INTO RESPONSE CTLR SAID 'ACFT X; MAKE AN IMMEDIATE L TURN.' I SAW THE WHITE JET ABOUT MY 3 O'CLOCK POS ABOUT BELLY UP. AFTER CLR OF CONFLICT WE CLBED FROM FL340 BACK TO FL350 AND ADVISED CTLR. NO EXPLANATION WAS OFFERED. WE DID HEAR CTLR TALK TO AN ACR AND SAID HE HAD 'STRAYED FROM HIS HOLDING PATTERN.' I AM REASONABLY CONVINCED THE TCAS SAVED US ON THIS DAY. A WHITE ACFT AGAINST A WHITE CLOUD HORIZON WOULD HAVE BEEN VERY DIFFICULT TO DETECT WITH ENOUGH TIME TO TAKE EVASIVE ACTION. CLOSURE RATE WAS SUPER FAST. I THINK OF THE TIMES WE FLEW WITH TCAS INOP AND I'M SURE GLAD TODAY WAS NOT ONE OF THOSE DAYS. SUPPLEMENTAL INFO FROM ACN 676165: AT CRUISE ALT FL350 PASSING AGARD INTXN ON J222; RECEIVED TA FOLLOWED ALMOST IMMEDIATELY BY A 'DSND' RA. I DSNDED ACFT TO FL340. FO NOTICED AN ACFT BELLY PASSING FROM THE NOSE DOWN THE R SIDE OF THE ACFT SLIGHTLY HIGH. ZDC CALLED FOR AN 'IMMEDIATE L TURN TO 320 DEGS' AND WE COMPLIED. AFTER STARTING TO RETURN TO THE ASSIGNED ALT AND COURSE; CTR CTLR TOLD AN ACR TO TIGHTEN UP HIS HOLDING PATTERN AS HE WAS STRAYING FROM HOLDING AIRSPACE. WX WAS VFR WITH A BRILLIANT WHITE UNDERCAST AND VERY BRIGHT SUNLIGHT. WE COULD NOT SEE THE ACFT AT THE TA AND ONLY PICKED HIM UP WHEN WE WERE BELOW THE FLT LEVEL.