MLG SLIDES ONTO AN ACTIVE RWY THEY HAD BEEN TOLD TO HOLD SHORT OF. VISIBILITY WAS REDUCED AND BY THE TIME THE CAPT REALIZED HOW CLOSE TO THE RWY HE WAS; IT WAS TOO LATE TO STOP ON THE SLIPPERY SNOW COVERED TXWY.

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AFTER LNDG RWY 12L AT STL; FLT WAS CLRED BY GND CTL TO 'HOLD SHORT RWY 12R ON TXWY R.' TXWYS/RWYS/RAMPS WERE SNOW COVERED UP TO 3-4 INCHES; WITH LIGHTS/MARKINGS/SIGNS OBSCURED BY SNOW. VISIBILITY WAS 1/2 MI IN SNOW. ACFT WERE LNDG RWY 12L; DEPARTING RWY 12R. I LOOKED DOWN TO CONFIGURE A COM RADIO; AND HEARD THE CAPT SAY 'IS THAT THE RWY?' I LOOKED UP TO SEE THAT THE NOSE OF OUR ACFT WAS WELL ON THE RWY (12R); PERHAPS AS MUCH AS 30-40 FT PAST RWY EDGE. I CONFIRMED THAT WE HAD GONE ON THE RWY. THE CAPT THEN APPLIED PWR TO THE ENGS TO CLR THE ACFT FROM THE RWY. I CALLED GND CTL AND STATED WE HAD CROSSED RWY 12R; AND WERE CLR. I RECEIVED NO REPLY FROM GND CTL AND CALLED AGAIN. THEY REPLIED BY CLRING US TO OUR RAMP; WITH NO FURTHER REQUESTS/INSTRUCTIONS. THE CAPT STATED THAT WHEN HE ATTEMPTED TO STOP SHORT OF RWY 12R; THE ACFT SLID ONTO THE RWY. I COULD HAVE HELPED AVOID THIS SIT BY NOT LOOKING DOWN AS WE APCHED RWY 12R. THE FACT THAT STL IS OUR DOMICILE ARPT MAY HAVE LED TO A FAMILIARITY THAT BRED COMPLACENCY. HAD WE BEEN AT AN UNFAMILIAR ARPT; WE WOULD LIKELY HAVE BEEN MORE ALERT TO SURFACE CONDITIONS; TXWY SLOPE; AND HOLD SHORT LINES.