A WDB FLC LANDED ON THE WRONG RWY. UNAUTH LNDG UNAUTH RWY WRONG RWY LNDG.

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ARRIVING FROM THE W OF STL ON THE TRAKE ARR; WE RECEIVED ATIS INFO; LNDG RWYS 30L AND R. INITIAL CONTACT WITH STL APCH; WE WERE GIVEN DEPART SNYDR INTXN ON A HDG OF 125 DEGS AND WE UNDERSTOOD RADAR VECTORS FOR RWY 30L. I THOUGHT I ACKNOWLEDGED RWY 30L. AS OUR CREW PROCS REQUIRE; WE BRIEFED FOR RWY 30L. WE WERE GIVEN A CLOSE TURN ON AND HAD TO DSND AT A HIGHER THAN NORMAL RATE TO INTERCEPT THE LOC. WE INTERCEPTED THE LOC ABOUT 4 MI OUTSIDE THE OM AND SWITCHED OVER TO ST LOUIS TWR. CHKED IN OVER THE OM AND WERE CLRED TO LAND. OVER THE APCH END OF RWY 30L; WE NOTICED AN AIRLINE B737 XING THE RWY ABOUT 8000 FT DOWN THE RWY. HE CLRED THE RWY COMPLETELY BEFORE WE TOUCHED DOWN. AFTER CLRING THE RWY; WE CHANGED TO GND CTL AND TAXIED TO THE GATE AND NOTHING WAS EVER SAID THAT WE WERE LINED UP WITH; OR LANDED ON THE WRONG RWY. AT THE GATE; RAMP CTL SAID ST LOUIS TWR WANTED ME TO CALL THEM. THE TWR SUPVR ADVISED ME THAT I LANDED ON THE WRONG RWY AND THAT HE WAS GOING TO REVIEW THE TAPES AND HOPED NO ACTION WOULD BE TAKEN. FROM INITIAL CONTACT; I THOUGHT AND READ BACK RWY 30L FOR LNDG. WE HAD A HARD TIME DSNDING TO THE LOC ALT. WHEN HE CLRED US TO INTERCEPT THE LOC; I DO NOT REMEMBER HIM STATING WHICH RWY. AT THE OM; IF TWR SAID RWY 30R; WE DID NOT CATCH IT. NOW THIS IS AT LEAST 3 TIMES THAT THE CORRECT RWY WAS OR SHOULD HAVE BEEN STATED. HOW WE DIDN'T HEAR OR UNDERSTAND THE CORRECT RWY; I HAVE NO IDEA OR CORRECTIVE ACTION TO KEEP THIS FROM HAPPENING AGAIN. WE WILL JUST HAVE TO BE MORE CAREFUL WITH OUR COM.