ACR MLG FREIGHTER CLBS THROUGH OCCUPIED ALT ON A 'MAINTAIN VISUAL ON TFC' CLRNC. FAILS TO DO SO AS FLC MISJUDGES SPD AND SEPARATION DISTANCE IN THE DARK. NMAC OCCURS.

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WE WERE AT 14000 FT MSL FLYING DIRECT TO THE QUITMAN; TX; VOR. OUR POS WAS AROUND 85 DME ON THE 220 DEG RADIAL FROM QUITMAN VOR. OUR SPD WAS AROUND 300 KIAS. ZFW HAD STOPPED OUR CLB TO FL230 AT 14000 FT BECAUSE OF TFC. ZFW SAID OUR TFC WAS AN ATR AT 11 O'CLOCK; 10-12 MI LEVELING AT 15000 FT MSL. WE CALLED THE TFC IN SIGHT EVEN THOUGH WE DID NOT SEE THE ACTUAL ACFT BUT THE ATR'S WHITE STROBE LIGHT WAS EASILY VISIBLE. THE ATR WAS CLOSING OUR POS BUT DID NOT APPEAR TO BE GOING VERY FAST. CTR SAID TO MAINTAIN VISUAL WITH THE TFC AND CLB TO FL230. CTR ALSO ADDED WE MIGHT WANT TO COME 5 DEGS L TO GIVE US MORE SEPARATION TO GO BEHIND THE ATR. WE DETERMINED THE ATR WAS 6-8 MI AWAY. WHILE MAKING THE HDG CHANGE OF 5 DEGS AND THE PF ADDED CLB PWR; WE DETERMINED TO TRADE OUR EXCESS AIRSPD FOR AN INCREASED RATE OF CLB TO INSURE THAT WE WOULD BE TO AT LEAST 16000 FT AS FAST AS POSSIBLE. IN OUR CLB; THE ATR MOVED TO OUR 12 O'CLOCK POS. THE ATR PASSED L TO R (HEADED SE) ABOUT 1 O'CLOCK TO OUR NOSE AND SLIGHTLY HIGH. OUR ALT WAS APPROX 15500 FT MSL AND CLBING. SINCE IT WAS DARK BY THIS TIME; IT WAS DIFFICULT TO DETERMINE THE ATR'S ACTUAL DISTANCE; TRACK AND RATE OF CLOSURE. THE PRUDENT DECISION WOULD HAVE BEEN FOR US TO STAY AT 14000 FT MSL UNTIL THE TFC HAD PASSED AND WAIT FOR ATC TO DETERMINE THAT PROPER IFR SEPARATION HAD OCCURRED AND CLB ONLY WHEN ATC ISSUED A FURTHER CLB CLRNC. SUPPLEMENTAL INFO FROM ACN 368354: THE CLRNC STATED TO MAINTAIN VFR SEPARATION AND THAT A L TURN SHOULD KEEP US CLR OF THE TFC. I TURNED L 10 DEGS AND INCREASED THE RATE OF CLB TO PASS THROUGH 15000 FT AS RAPIDLY AS POSSIBLE. AT APPROX 16000 FT; WE PASSED IN CLOSE PROX TO AN ATR. THE ATR HAD CLBED BECAUSE OF A TCASII WARNING (THIS WAS THE 15000 FT TFC). AFTER THIS EVENT; I HAVE DECIDED NOT TO ACCEPT ANY MORE VFR CLB CLRNCS AT NIGHT DUE TO THE DIFFICULTY OF DETERMINING THE DIRECTION OF THE OTHER ACFT AND THE DISTANCE FROM THE OTHER ACFT.