UNAUTH TXWY ENTRY ON LNDG PROC ROLLOUT.

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ON SEP/SUN/93; WE WERE ASKED TO FERRY AN ACFT TO PDX FOR ROUTINE MAINT. THE CAPT; DURING PREFLT; TALKED WITH FLT CTL TO GET INFO ON WHERE TO GO ONCE REACHING PDX. THERE WAS NO INFO AS TO OUR DEST ON THE PDX AIRFIELD. THE CAPT AND I AGREED TO FLY THE TRIP AND COORDINATE WITH OUR OPS AT THE PDX ARPT ITSELF. ENRTE; WE CAME TO FIND OUT THAT PDX OPS DIDN'T KNOW EXACTLY WHERE OUR MAINT FACILITY WAS ON THE FIELD. ALSO; ENRTE THE CAPT AND I BRIEFED THE APCH; AND ARPT DIAGRAM FOR A GENERAL IDEA OF WHERE TO GO ONCE ARRIVING AT THE ARPT. AFTER LNDG ON RWY 28L; THE CAPT BEGAN TO CLR THE ACTIVE RWY ON A HIGH SPD TXWY WHICH APPEARED TO BE THE BEST OPTION TO GET WHERE WE NEEDED TO GO ON THE ARPT. SITTING JUST OFF OF TXWY FOXTROT WERE ACR Y HVY ACFT; ALONG WITH SOME WDBS. THIS ADDED TO THE ILLUSION THAT AN MLG WOULD BE FINE TO TAXI ON THE FULL LENGTH OF TXWY F. THE CAPT PULLED ONTO F JUST ENOUGH TO CLR THE RWY. I CALLED GND CTL TO GET PROGRESSIVE TAXI TO OUR DEST. WE WERE THEN INFORMED BY GND CTL THAT 'FOR FUTURE REF; THAT PORTION OF THE TXWY WAS NOT AVAILABLE TO US; BUT TO CONTINUE.' AT THAT POINT WE WERE INFORMED THERE WAS PLENTY OF ROOM TO REVERSE COURSE; OR BE TOWED CLR WITHOUT COVERING ANY OF THE RESTR TXWY. WE WERE CERTAINLY NOT INTENTIONALLY NEGLIGENT. THE CAPT MADE THE BEST CHOICE AVAILABLE AT THE TIME; TO NOT LINGER ON AN ACTIVE RWY. IT SEEMS STRANGE THAT A WT RESTR TXWY WOULD BE PUT AS A HIGH SPD EXIT ON A BUSY RWY AT A MAJOR ARPT. I FEEL THAT SUCH A TXWY COULD BE MUCH BETTER MARKED. WE HAVE BOTH LEARNED FROM OUR EXPERIENCE; AND I KNOW THAT I WILL NEVER TAKE FOR GRANTED ANY TXWY AGAIN.