ACFT Y LANDED ON OCCUPIED RWY; HAD LTSS FROM ACR X. PLTDEV. SYS ERROR.

Narrative

DURING THE LNDG ROLLOUT RWY 34R AT SEA-TAC ARPT THE TWR ADVISED THAT THERE WAS AN ACFT 1 MI ON FINAL BEHIND US FOR THE SAME RWY AND TO EXPEDITE THE TURNOFF AT 'N' TXWY AND CONTACT GND TURNING THE CORNER. THIS WAS THE FIRST INDICATION THAT THERE WAS AN ACFT SO CLOSE BEHIND FOR MY RWY. WE WERE ALSO INFORMED OF THE MD-11 TAXIING N ON THE 'B' TXWY (S OF 'N') WOULD BE HOLDING SHORT. AS WE LEFT THE RWY AT 'N;' IT WAS APPARENT THAT THE MD-11 DID NOT HOLD SHORT. TWR/GND SUBSEQUENTLY ADVISED THAT THEY WOULD BE TAXIING TO THE R-HAND SIDE.NOW AT 90 DEGS TO THE RWY; IT WAS APPARENT THAT THE MD-11 WAS NOT HOLDING SHORT AS INITIALLY EXPECTED NOR SUFFICIENTLY TO THE R SIDE OF THE TAXI/RAMP AREA. I STOPPED THE ACFT AND DIRECTED THE FO TO INFORM THE TWR THAT WE ARE UNABLE TO CLR. (AS I COULD NOW VIEW THE APCHING ACFT ON FINAL). THIS CALL WENT OUT OVER GND CTL RATHER THAN TWR; NOT REALIZING THAT THE FO HAD ALREADY SWITCHED. IT BECAME APPARENT THAT ACFT Y ARRIVING SHOULD GAR AND I SUBSEQUENTLY DIRECTED THE FO TO TELL THE TWR TO HAVE Y GAR. THE FO INDICATED THE ARRIVING Y WAS IN THE LNDG TRANSITION. AT THIS TIME I ATTEMPTED TO MOVE THE ACFT AHEAD AS MUCH AS SPACE WOULD PERMIT AS THE MD-11 PASSED EXIT. IN THE END OF IT; THE ARRIVING ACFT TOUCHED DOWN AND COMPLETED THE LNDG. A GOOD PORTION OF OUR ACFT WAS STILL OVER THE HOLD SHORT LINE. PROBS: Y DID NOT PROPERLY SPACE ITSELF WHEN INFORMED THAT HIS ACFT WAS 40 KTS FASTER.