B737 CREW HAD A GAR WHEN A TURBOPROP CROSSED ONTO THE LNDG RWY AHEAD OF THEM AT DCA.

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DCA WAS ACCEPTING VISUAL APCHS FROM THE RIVER VISUAL PROC TO RWY 19. THIS WAS OUR SECOND TRIP SEGMENT FROM IAH TO DCA THIS WK. I WAS HAND FLYING THE APCH AND MY FO AND I HAD FLOWN 6 FLT SEGMENTS TOGETHER ALREADY FOR THE MONTH OF MAY. WE WERE A LITTLE FATIGUED FROM THE DAY'S FLYING AND NOTED IT ENRTE TO EACH OTHER. I HAD ABOVE AVERAGE CONFIDENCE IN HIS SKILLS AND CRM AND AS IT TURNED OUT; FATIGUE AND HUMAN FACTORS WERE HANDLED VERY WELL. WE HAD TO PERFORM A GAR AT ABOUT 200 FT AGL DUE TO A RWY INCURSION BY WHAT APPEARED TO BE A COMMUTER TURBOPROP; BUT I'M NOT SURE OF THE TYPE. TURNING FINAL TO LINE UP WITH RWY 19 ALL WAS NORMAL. TFC VOLUME SEEMED BUSY. WHILE DSNDING ON THE VASI GLIDE PATH; I SAW THE ACFT; PERHAPS ON TXWY F; MOVING L TO R; BEGINNING TO ENCROACH ONTO THE RWY. THE TWR CTLR FIRST ATTEMPTED TO CONTACT THAT ACFT; STATING; '(CALL SIGN?)...VERIFY HOLDING SHORT OF RWY 19.' AT THIS POINT; WE INITIATED A GAR. THE CTLR; WITHOUT WAITING FOR AN ANSWER FROM THE ENCROACHING ACFT; EMPHATICALLY ORDERED A GAR IMMEDIATELY...VERY WELL DONE ON HIS PART TO MAKE SURE WE HEARD HIM. INSTRUCTIONS FOLLOWED FOR VECTORS FOR ANOTHER APCH; WHICH ENDED WITHOUT INCIDENT. THIS ARPT MUST INVEST IN EFFECTIVE HOLD SHORT LIGHTING TO HELP PREVENT THESE INCURSIONS. TXWYS C; F; J...WHERE THEY CROSS RWYS; SHOULD GET SOMETHING IN THE WAY OF LIGHTING; NOW. THE WORKLOAD; ON THE TWR WITH 3 XING RWYS; IS TOO HIGH. HE IS UNABLE TO HANDLE SOME DISTRACTIONS BECAUSE HE IS BOTH GND CTL AND TWR CTL DUE TO XING RWYS. SUPPLEMENTAL INFO FROM ACN 472473: THE TFC AT DCA IS ALWAYS HVY AND THE TWR AND GND FREQS ARE BUSY. A FLASHING LIGHT AT THE HOLD SHORT LINES MIGHT BE HELPFUL. ALSO USING ONLY 1 RWY FOR LNDG AT NIGHT WOULD STOP THE ACFT FROM HAVING TO CROSS RWY 19 TO GET TO THE TERMINAL. ANOTHER FACTOR THAT MAY HAVE CONTRIBUTED TO THE RWY INCURSION IS THE RIVER VISUAL APCH TO RWY 19. SINCE WE ARE NOT LINED UP WITH THE RWY UNTIL SHORT FINAL; THERE IS LESS CHANCE FOR THE ACFT ON THE GND TO SEE US ON FINAL. I ALSO THINK FOLLOWING A RIVER AT NIGHT FOR A VISUAL APCH IS RISKY AT BEST.