B757 PIC MAKES EVASIVE DSCNT MANEUVER TO AVOID COLLISION WITH UNKNOWN ACFT XING OVERHEAD. PIC BELIEVES CTLR DID NOT TAKE INTO ACCOUNT B757 CLB RATE; XING TFC XOVER POINT; AND DECREASING SPACING BTWN LNDG AND DEP TFC.

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THE NEAR MISS TOOK PLACE ON TKOF OUT OF PDX WHERE WE CAME CLOSE TO STRIKING ANOTHER ACFT. WHEN PASSING APPROX 750 FT; THE TWR GAVE US A TFC RPT 'ACFT Y XING MIDFIELD AT 1000 FT.' I TRIED TO LEVEL THE ACFT WHICH WAS CLBING GREATER THAN 6000 FPM AND SCANNED FOR TFC. I SAW AN ACFT FLASH OVERHEAD. OUR ALT WAS ABOUT 850-900 FT MSL. IF I HAD HESITATED ON LEVELING THE ACFT; I WOULD NOT BE HERE TO WRITE THIS RPT. I'M NOT SURE WHY THE TWR HAD VECTORED THE ACFT OVER THE FIELD AT THIS ALT; BUT CONSIDER THIS ACTION TO BE VERY DANGEROUS. I DON'T THINK THE TWR ANTICIPATED THE PERFORMANCE OF OUR NEAR EMPTY ACFT WHEN DOING THIS. THE RESULT OF THIS MISJUDGEMENT WAS A NEAR MISS. I THINK THERE WAS ANOTHER FACTOR. THAT BEING; WE WERE IN POS WAITING FOR TKOF CLRNC; AND THERE WAS AN ACFT Z ON A 3 MI FINAL. MAYBE HE FELT PRESSURED TO CLR US FOR TKOF. IF HE HAD HELD US ON THE RWY THE OTHER ACFT WOULD HAVE HAD TO GO AROUND. FOR US THIS IS A VERY CRITICAL PHASE OF FLT WITH A LOT OF MODE CHANGES; CALLOUTS; CONFIGN CHANGES; AND FREQ CHANGES. OUR ATTN IS USUALLY STRAIGHT AHEAD; NOT TO THE SIDE. IN THE FUTURE I WILL HAVE TO ADD THIS AREA TOO. ANOTHER FACTOR IS CARGO ACFT ARE NOT REQUIRED TO HAVE TCASII. THIS WOULD HAVE BEEN A GREAT HELP!