ACR LGT PIC RPTS ON TCASII RA FROM SAME DIRECTION TFC FROM THE REAR SETTING OFF TCASII RA. ALTDEV ALT EXCURSION.

Narrative

APCH WAS EXTREMELY BUSY HAVING JUST COME OUT OF A HOLD BECAUSE OF TFC SATURATION. WAS BEING VECTORED FOR THE 31L APCH. WAS VECTORED THROUGH THE LOC AND THEN THROUGH THE 31R LOC (WE WERE AT 3000 FT). WAS FOLLOWING A FOREIGN CARRIER WDB. APCH ASKED IF WE COULD INTERCEPT THE 31R LOC. ANSWERED IN THE AFFIRMATIVE BUT WAS NOT TOLD TO INTERCEPT. CTLR THEN GAVE INSTRUCTIONS TO A COMMUTER TO SWITCH TO THE L RWY. ABOUT THE TIME WE CROSSED THE 31R LOC WE RECEIVED AN RA WITH A DSND COMMAND. COMPANY POLICY IS TO LEAVE THE TCASII IN TA/RA MODE UNTIL 2000 FT AND THE REQUIREMENT TO RESPOND TO ALL COMMANDS UNLESS DOING SO WOULD FURTHER DECREASE SAFETY. DSCNT WAS BEGUN AND AN INTERCEPT OF THE LOC WAS ALSO INITIATED AFTER GETTING AN ALL CLR ADVISORY. IN RPTING TO APCH WHAT WE HAD DONE THEY CONFIRMED THAT THEY WANTED US TO DO THE INTERCEPT OF THE 31R LOC AND THAT THE RA HAD BEEN CAUSED BY THE COMMUTER SWITCHING RWYS; KEEPING HIS SPD UP AND XING BEHIND US WITH US IN SIGHT TO EXPEDITE TFC FLOW. THIS IS THE SECOND TIME IN THE LAST 6 MONTHS THAT THE IDENTICAL SIT HAS OCCURRED DURING APCH. THE OTHER OCCURRENCE WAS AT BNA WITH AN ACFT XING BEHIND US AT HIGHER SPD GOING TO THE PARALLEL RWY. TCASII IS GREAT BUT IT CAN BE A PAIN WHEN THE ACFT IS CONVERGING FROM THE REAR AND HAS YOU IN SIGHT FOR A VISUAL APCH. WE NEED MIRRORS IN THE LGT.