A B737 CREW INSTRUCTED POS AND HOLD BEHIND AN RJ DEPARTING; ADVISES DFW TWR OF AN MD80 ON A SHORT FINAL FOR THE SAME RWY.

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AFTER A LENGTHY GND DELAY WE WERE #1 FOR TKOF HOLDING SHORT OF RWY 35L. AN RJ WAS HOLDING IN POS ON RWY 35L. THE FO AND I BOTH NOTICED A JET ON FINAL APCH A COUPLE MI OUT; HOWEVER; SINCE IT WAS NIGHTTIME IT WAS DIFFICULT TO TELL IF IT WAS OUR RWY (35L) OR A PARALLEL RWY THE JET WAS LINED UP FOR. AFTER SCANNING MY TCASII DISPLAY AND LOOKING BACK OUT THE WINDOW IT THEN BECAME OBVIOUS TO BOTH THE FO AND I; THE JET WAS LINED UP FOR RWY 35L. ABOUT THIS TIME TWR CTLR CLRED THE RJ FOR TKOF. WE WERE THEN CLRED FOR POS AND HOLD ON RWY 35L WITH THE JET ON ABOUT A MI FINAL. WITHOUT MOVING OUR ACFT OR RELEASING THE PARKING BRAKE MY FO TOLD DFW TWR LOOKS LIKE A PLANE ON SHORT FINAL RWY 35L. THE CTLR THEN IMMEDIATELY ORDERED ACR Y TO GO AROUND WHILE STATING THEY WERE LINED UP ON THE WRONG RWY. THE SPACING LOOKED TIGHT AS THE RJ WAS JUST ROTATING WHILE THE MD80 STARTED THEIR GAR. THE CTLR NEVER FOLLOWED UP WITH A HOLD SHORT CLRNC FOR US ALTHOUGH IT MAY HAVE BEEN OBVIOUS AT THIS POINT WE WEREN'T GOING TO TAKE THE RWY. AFTER A WHILE WE WERE CLRED FOR TKOF AND HAD A ROUTINE FLT TO ORD. THE SAFETY CHAIN BROKE DOWN SIGNIFICANTLY WITH THIS INCIDENT. HAD WE ACCEPTED THE POS AND HOLD CLRNC WITHOUT CHKING THE FINAL APCH; A MAJOR HULL LOSS WITH OUR B737 AND THE MD80 WAS VERY POSSIBLE. IT WAS CLR TO US THE CTLR NEVER KNEW THE MD80 WAS LINED UP ON THE WRONG RWY UNTIL WE INTERVENED ON THE RADIO. HAD THE CTLR NOT CLRED THE RJ FOR TKOF; OR CLRED THE RJ AT A LATER POINT IN TIME; A HULL LOSS BTWN THE MD80 AND THE RJ WAS ALSO VERY POSSIBLE. I ALSO WONDERED WHY THE PNF ON THE MD80 ALLOWED THE PF TO LINE UP AND CONTINUE APCH TO THE WRONG RWY. ALSO; WHY DIDN'T DFW APCH CTL NOTIFY THE MD80 OR THE TWR OF THE WRONG LINE-UP. WITHOUT OUR RADIO CALL BREAKING THE ERROR CHAIN THIS COULD HAVE TURNED OUT VERY BAD INDEED. THIS WAS SIMPLY CUTTING THINGS MUCH TOO FINE FOR A SAFE OP.