MLG CREW HAD RWY INCURSION AT STL.

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THE FO CALLED STL GND FOR TAXI OUT REQUEST. WE RECEIVED CLRNC AND INSTRUCTIONS TO TAXI TO RWY 12L VIA TXWY D; RWY 6 AND HOLD SHORT OF RWY 12R. WE HELD SHORT OF RWY 12R. WE WERE THEN GIVEN CLRNC TO CONTINUE TAXIING TO RWY 12L. AS I WAS SLOWING THE ACFT TO STOP ON RWY 6; STL TWR TOLD US TO HOLD OUR POS AND INFORMED US WE HAD CROSSED THE HOLD SHORT LINE. AFTER SEVERAL SECONDS; STL TWR THEN DIRECTED AN ACFT ON FINAL TO GO AROUND. NEITHER THE FO NOR MYSELF SAW THE HOLD SHORT LINE ON RWY 6 UNTIL TWR MENTIONED IT TO US. WE COULD THEN SEE IT RIGHT BENEATH US AFTER WE STOPPED. THE PAINTED LINES ON RWY 6 APPEARED TO NEED REPAINTING. RWY 12L DOES NOT PHYSICALLY INTERSECT WITH RWY 6; AND SO WE WERE NOT EXPECTING TO SEE HOLD SHORT LINES ON RWY 6. ANOTHER ACFT WAS TAXIING ABEAM US ON TXWY C TO RWY 12L. WE HAD THE IMPRESSION THAT WE WERE USING RWY 6 AS A TEMPORARY TXWY TO BYPASS THE TFC ON TXWY C (THE NORMAL TAXI RTE TO RWY 12L). DUE TO THIS PERCEIVED TAXI 'PLAN' AND THE FADED HOLD SHORT LINES; WE DID NOT SEE OR EXPECT TO SEE; THE HOLD SHORT LINES ON RWY 6.