B757 WITH ZTL IN HOLDING AT FL260 RECEIVED TCASII TA AND VISUALLY OBSERVED ANOTHER ACFT AT THE SAME ALT; SUBSEQUENTLY RESPONDING TO A TCASII RA.

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ENRTE TO ATL FROM IAD AS B757; ALMOST 4 HRS LATE DUE TO ZDC WX HOLDS. LONG DELAYS COMBINED WITH A COLD FRONT AND ASSOCIATED LINE OF TSTMS; CREATED A NIGHTMARE SCENARIO WITH AIR TFC. WE WERE ASSIGNED FL260 DURING A STEP-DOWN DSCNT AND ISSUED HOLDING INSTRUCTIONS FOR MACEY AS PUBLISHED EXCEPT FOR L TURNS DUE TO TSTM ACTIVITY. WE WERE IN LEVEL FLT IN OUR SECOND TURN IN HOLDING INBOUND TO MACEY WHEN THE TCASII GAVE US A TA AT 10 O'CLOCK POS WHICH WE ACQUIRED VISUALLY. AS WE WATCHED; THE TA BECAME AN RA AND COMMANDED A DSCNT. THE GREEN ARC ON THE VSI WAS VERY SMALL; SO IT DID NOT APPEAR THAT WE WERE IN IMMEDIATE JEOPARDY. ABOUT THE TIME ZTL ISSUED A 'CLB IMMEDIATELY' CALL TO B757; I DISENGAGED THE AUTOPLT AND BEGAN A VERY GRADUAL DSCNT WITHIN THE PARAMETERS OF THE RA. I GLANCED AT THE RADAR SCREEN AND NOTED A +200 FT READING ON THE SUBJECT ACFT. WE WERE VISUALLY 'PADLOCKED' ON THE OTHER ACFT AND IT WAS OBVIOUS THAT THERE WOULD BE NO COLLISION. ZTL ISSUED US DSCNT INSTRUCTIONS TO FL240 AND THE COPLT ADVISED THAT WE WERE ALREADY DSNDING IN RESPONSE TO AN RA. THIS CAME AT THE END OF A VERY LONG (15+ HRS) AND STRESSFUL DAY. IN THE DAYS BEFORE TCASII; THE OUTCOME COULD HAVE BEEN MUCH WORSE. THE SYS WORKED EXACTLY AS DESIGNED AND AVOIDED THE NEED FOR VIOLENT MANEUVERING. THE CTLR GAVE ME A NUMBER TO CALL TO SPEAK WITH A SUPVR; AND COMMENTED THAT IT WAS THEIR MISTAKE. I CALLED THE SUPVR AFTER LNDG; MAINLY TO PASS WORD TO THE CTLR THAT WE HELD NO HARD FEELINGS AND THAT HE SHOULDN'T 'BEAT HIMSELF UP' TOO BADLY. HE WAS INUNDATED WITH TFC; LATE AT NIGHT; WITH BAD WX; MISSED AND BLOCKED RADIO CALLS; AND UNDER A GREAT DEAL OF STRESS. TCASII IS WORTH EVERY PENNY. IT ADDS THE EXTRA LAYER OF PROTECTION NEEDED WHEN THERE IS HUMAN ERROR BY AN OVER-WORKED PROFESSIONAL. IF WE HAD BEEN IMC AND HIS RADIO CALLS HAD BEEN BLOCKED; TCASII WOULD HAVE SAVED OUR BACON.