SA227 TAXIES ACROSS RWY 20 AT SGF AS ACFT Y IS ON APCH; RESULTING IN ACFT Y EXECUTING A GAR.

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AFTER LNDG ON RWY 2 AT BRANSON ARPT; SPRINGFIELD; MO; WE EXITED THE RWY AT TXWY B AND TAXIED STRAIGHT AHEAD INTO A SPACE ON THE CARGO RAMP BETWEEN A B727 AND AN MD80. AFTER LOADING OUR FREIGHT WE COMPLETED A U-TURN TO EXIT THE PARKING SPOT AND THE FO CALLED GND CONTROL FOR TAXI CLRNC. CLRED TO RWY 20; I TAXIED THE SAME PATH TOWARD THE RWY THAT WE HAD FOLLOWED COMING IN WITH THE INTENTION OF PICKING UP THE YELLOW TAXI LINE AT TXWY U WHERE I WOULD MAKE A L TURN TOWARD THE APCH END OF THE RWY. COINCIDENTALLY; THIS PATH POINTED THE ACFT DIRECTLY AT TXWY B AND THE RWY. AS WE PROGRESSED THE SHORT DISTANCE FROM THE PARKING AREA TO THE TXWY; I CALLED FOR FLAPS 1/4 AND TAXI CHKLIST. AS THE FO CALLED OFF THE CHKLIST I DIVIDED MY ATTENTION BETWEEN MONITORING OUR PROGRESS ON THE GND AND CHECKLIST THE ACFT CONFIGURATION ITEMS ON THE CHKLIST. I OBSERVED A TAXI LINE AND FOLLOWED IT CONTINUING TOWARD THE RWY. I GLANCED AHEAD TO SEE THE TAXI LINE BENDING L AS I EXPECTED. BECAUSE THE TXWY IS PART OF THE CARGO RAMP IN THIS AREA I BEGAN TO LET THE NOSE DRIFT L APPROX TEN DEGS TO INTERCEPT THE TXWY THEN REDIRECTED MY ATTENTION TO THE COCKPIT. AS THE ACFT BEGAN TO TURN; THE FO LOOKED UP FROM THE CHKLIST AND STATED 'THAT'S THE RWY.' I IMMEDIATELY STOPPED THE ACFT AND CONFIRMED HIS OBSERVATION. APPARENTLY WHILE DISTRACTED INSIDE THE AIRPLANE WITH THE CHKLIST AND BELIEVING I WAS STILL ON THE CARGO RAMP; I DID NOT NOTICE THAT I HAD PASSED OVER THE INTERSECTION OF THE TAXI LINES FOR TXWYS U AND B AND CONSEQUENTLY MISSED THE TURN ON TO TXWY U. THE CURVE IN THE TAXI LINE THAT I WAS NOW SEEING WAS ACTUALLY THE TAXI LINE LEADING OFF THE RWY TO TXWY B AND I HAD OBVIOUSLY CROSSED THE HOLD SHORT LINE. I HAD MADE CASUAL NOTE OF AN ACFT ON FINAL APCH AS WE BEGAN TO TAXI AFTER COMPLETING THE TURN OUT OF PARKING. THIS ACFT NOW BECAME THE CRITICAL FACTOR IN MY ASSESSMENT OF THE SIT AND MY FINAL COURSE OF ACTION. AT THIS POINT I ESTIMATE THE LNDG ACFT'S DISTANCE AT APPROXIMATELY A 1/2 MILE FINAL. I SURMISED THREE ALTERNATIVES. I COULD ATTEMPT TO BACK UP BUT DUE TO THE ACFT'S POWER CONFIGURATION; THE ACFT WEIGHT; AND LEAST OF ALL GIVEN THE SIT; A COMPANY POLICY PROHIBITING IT; THIS OPINION WAS REJECTED. I LOOKED OUT THE LEFT SIDE WINDOW TO SEE IF I HAD ROOM TO TURN-AROUND AND OBSERVED THAT I HAD CROSSED THE HOLD SHORT LINE APPROX ONE ACFT LENGTH. THIS POSITION TECHNICALLY PUT THE ACFT ON THE RWY; BUT NOT ENOUGH TO ENSURE A SUCCESSFUL L TURN-AROUND WITHOUT POSSIBLY LEAVING THE PAVED AREA AND STRIKING AN OBJECT SUCH AS A TXWY LIGHT WITH THE PROP OR GETTING STUCK. WITHOUT KNOWING THE SIZE OR CLOSURE SPEED OF THE LNDG ACFT; I WAS ALSO CONCERNED ABOUT THE TIME IT WOULD TAKE TO MAKE SUCH A TIGHT TURN AROUND AND WAS AFRAID I MIGHT MAKE MATTERS WORSE BY ACTUALLY ENCROACHING FURTHER ONTO THE RWY DURING THE MANEUVER. I COULD NOT SEE OUT TO THE R SIDE OF THE ACFT TO ASCERTAIN THE POSSIBILITY OF TURNING IN THAT DIRECTION. BECAUSE OF THE APCHING ACFT; I DID NOT FEEL THERE WAS TIME TO DISCUSS THE MATTER WITH MY FO SO I REJECTED THIS ACTION IN FAVOR OF THE THIRD OPTION. ACROSS THE RWY I NOTICED ANOTHER TXWY EXITING TO THE OPPOSITE SIDE OF THE RWY. JUDGING BY OBSERVING THE APCHING ACFT'S LNDG LIGHTS; I ESTIMATED THE APCHING ACFT'S DISTANCE AT APPROX 1/4 MILE AND DECIDED THE SAFEST AND MOST EXPEDIENT THING TO DO WAS TO APPLY POWER; CROSS THE RWY AND EXIT AS QUICKLY AS POSSIBLE. THAT IS WHAT I DID. THE TIME BETWEEN THE DISCOVERY OF MY MISTAKE; THE ASSESSMENT AND THE DECISION PROCESS TOOK ONLY A FEW SECONDS. HAD THERE BEEN NO TXWY AVAILABLE FOR ME TO CLR FROM ACROSS THE RWY; THEN I WOULD HAVE HAD TO ASK ATC TO INTERVENE OR POSSIBLY ANNOUNCE OUR POS TO THE ARRIVING ACFT ON THE TOWER'S FREQ. HOWEVER; AT THE TIME THE PRIMARY CONSIDERATION WAS TO CLR THE RWY IF POSSIBLE AND THERE WAS NO TIME TO CONSULT WITH ATC AS THERE MIGHT HAVE BEEN UNDER OTHER CIRCUMSTANCES. TURNING THE ACFT AROUND TO RETURN TO THE PROPER SIDE OF THE HOLD SHORT LINE MAY WELL HAVE BEEN A VIABLE COURSE OF ACTION. HOWEVER; WITHOUT ADEQUATE TIME TO CONSULT MY FO OR ATC; I FELT AS THE CAPT; I SHOULD TAKE ADVANTAGE OF WHAT I PERCEIVED TO BE THE SAFEST COURSE OF ACTION FOR BOTH THE LNDG ACFT AND FOR US. I DO NOT KNOW IF ATC ADVISED THE LNDG ACFT TO ABANDON ITS APCH OR IF THE PLT DID SO ON HIS OWN. WE WERE STILL ON GND CTL FREQ AND DID NOT HEAR ANY COMS BETWEEN THE TWR AND THE LNDG ACFT. AFTER CLRING THE RWY AND TURNING L TOWARD THE APCH END OF THE RWY WE DID OBSERVE THE LNDG ACFT PASS OVER THE APCH END OF THE RWY AND GAR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THEY WERE DISTRACTED WITH DOING THE CHKLIST AND BELIEVES THAT THIS LED TO THE RWY INCURSION.