AN ACR MDT WAS UNABLE TO HOLD SHORT OF A XING RWY. PLT DEV.

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WE WERE CLRED TO LAND ON RWY 24 AND TO HOLD-SHORT OF RWY 30L FOR LNDG TFC. CAPT READ THE CLRNC BACK AND I PROCEEDED TO CONTINUE THE APCH. I REMAINED ON LOC AND GS UNTIL APPROX 100 FT AGL AT WHICH TIME I STARTED TO SLOW THE ACFT TO LNDG SPD AS WELL AS DSND SLIGHTLY BELOW GS. I CARRIED TOO MUCH PWR IN THE FLARE AND WE FLOATED FOR A FEW SECONDS BEFORE TOUCHING DOWN. I IMMEDIATELY APPLIED FULL REVERSE AND MAX BRAKING. THE TWR CTLR QUESTIONED WHETHER WE WERE GOING TO HOLD SHORT AND CAPT ACKNOWLEDGED WITH 'YES; WE UNDERSTAND.' THE ACFT WAS SLOWED TO A SLOW TAXI SPD BY THE TIME I COULD SEE THE HOLD SHORT LINES FOR RWY 30L. CAPT ANNOUNCED 'MY ACFT;' I RELINQUISHED THE PWR LEVERS AND CAPT CTLED THEM. I HEARD A XMISSION TO TURN L BUT DID NOT HEAR TO WHOM IT WAS DIRECTED NOR A RESPONSE. CAPT THEN PUSHED THE PWR LEVERS FORWARD AND STARTED TO TURN THE ACFT L. I ASSUMED THAT I MISSED A XMISSION DIRECTED TO US. I THEN ASKED CAPT WHERE HE WAS GOING AND BY THEN HE HAD CROSSED THE HOLD SHORT LINE FOR RWY 30L. THE TWR CTLR THEN SAID 'YOU JUST CROSSED AN ACTIVE RWY.' CAPT THEN ADDED PWR AND CONTINUED ACROSS RWY 30L. I LOOKED UP TOWARD THE APCH END OF RWY 30L AND SAW A L-1011 EXITING THE RWY A FEW TXWYS UP THE RWY. I FEEL THAT THE OCCURRENCE COULD HAVE BEEN AVOIDED THROUGH BETTER COM. CAPT AND I WERE IN THE 11TH DUTY HR FOR THE DAY AS WELL AS THE 6TH FLT HR. I FEEL FATIGUE; POOR COM; AND POOR RWY MARKING AND LIGHTING WERE ALL CONTRIBUTING FACTORS IN THIS OCCURRENCE. I ALSO FEEL THAT THE HOLD SHORT OP AT NIGHT AT ST LOUIS SHOULD BE ABOLISHED DUE TO THE CONFUSION IT CAN CREATE AND THE DIFFICULTY IN DISCERNING CERTAIN RWY BOUNDARIES.