UNAUTH LNDG ON WRONG RWY IN A NIGHT OP.

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STL ATIS STATED THE WX WAS 2400 FT BROKEN 4400 FT OVCST VISIBILITY 7 LIGHT RAIN; 62 DEGS F; WIND 200 DEGS AT 4 KTS; ALTIMETER 29.95. NOTAMS: RWY 12L ILS OTS; RWY 12L APCH LIGHTS OTS. STL APCH INITIALLY GAVE US ILS RWY 12R. ON EXTENDED DOWNWIND APCH CHANGED THE RWY FOR US TO LDA RWY 12L. ON VECTOR TO FINAL WE RECEIVED A TCASII ALERT AT 2-3 O'CLOCK AT 1/2 MI AND 300-400 FT BELOW. WE WERE INSTRUCTED TO KEEP 170 KTS UNTIL POTSE (6.3 DME). THE TCASII STAYED AT SAME POS TO US. WHEN WE BROKE OUT OF THE CLOUDS; IT WAS RAINY; AND THE CEILING WAS RAGGED. WE STAYED ON THE LOC AND PICKED UP THE ARPT. RWY 12R HAD APCH LIGHTS. AT 3 DME I LINED UP ON WHAT I THOUGHT WAS RWY 12L. THE RWY LIGHTS WERE BRIGHT. IT WAS RWY 13 THOUGH. AS I APCHED THE RWY; I SAW AN ACFT ON THE RWY TAXIING TOWARD US. I ASKED TWR IF WE WERE CLRED TO LAND. HE SAID YES. I KEYED THE MIKE AND SAID 'I'M SORRY BUT THERE IS AN ACFT ON THE RWY.' I STARTED TO GAR; THEN THE ACFT CLRED THE RWY; SO I LANDED. FACTORS CAUSING THE PROB: THE CONFIGN SETUP OF RWY OPS. THE RWY 13 LIGHTS BEING BRIGHTER THAN RWY 12L OR RWY 12R. RWY 13 BEING ONLY 400-500 FT FROM RWY 12L. NO ILS ON RWY 12L. NO APCH LIGHTS ON RWY 12L. THE TCASII ALERT ALMOST ALL THE WAY IN. THE APCH END OF RWY 13 AND RWY 12L BEING ONLY 200-300 FT OFFSET. SUPPLEMENTAL INFO FROM ACN 316803: RECEIVED TCASII WARNING FROM ACFT AT 2-3 O'CLOCK; 400 FT BELOW. ACFT WAS ON ILS TO RWY 12R. SAW APCH LIGHTS FOR RWY 12R AND RWY LIGHTS FOR WHAT WE THOUGHT WAS RWY 12L. LIGHTS APPEARED TO BE UP TO FULL BRIGHT. WE DID NOT SEE ANY OTHER RWY LIGHTS. LINED UP ON RWY AND NOTICED THAT THERE WAS AN ACFT ON THE RWY. WE ASKED TWR IF WE WERE STILL CLRED TO LAND; WHICH WAS CONFIRMED. ACFT CLRED RWY AND WE LANDED ON RWY 13 THINKING IT WAS RWY 12L. ACFT WAS STOPPED WITH NO PROBS. FACTORS CAUSING THIS INCIDENT: FAILURE TO SEE RWY 12L RWY LIGHTS. CHANGE IN APCH ON DOWNWIND. APCH LIGHTS AND ILS RWY 12L OTS. REDUCED VISIBILITY. RWY LIGHTS UP FULL BRIGHT. ACFT ON APCH TO RWY 12R. CONCERN ABOUT ACFT ON RWY.