APU FAILED TO LIGHT ON FIRST ATTEMPT. SERIOUS TORCHING ON SECOND RESULTING IN SOME PASSENGER EVACUATING THROUGH OVERWING EXITS.

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INITIATED APU START AFTER CLRING RWY 12L; THEN CONTINUED WITH MY AFTER LNDG DUTIES. AT ST LOUIS THIS INCLUDES CALLING COMPANY RAMP AS SOON AS YOU LAND SO THEY CAN PLN AHEAD. APPROX 5-6 MINS LATER THE ACFT WAS TURNED TOWARD THE GATE; AT THIS TIME I CHKED MY FREQ AND VOLT METERS FOR THE APU GENERATOR; NEITHER GAUGE SHOWED ANY OUTPUT. I ANNOUNCED TO THE CAPT THAT ESSENTIAL PWR WAS SELECTED TO ENG #1 AND I WOULD NEED SOME TIME TO GET THE APU UP AND RUNNING. AS HAD HAPPENED SEVERAL TIMES IN THE PAST; I THOUGHT THAT THE SPRING LOADED APU TART SWITCH HAD SPRUNG DOWN PAST THE ON POS THEREBY NEVER ACTUATING A START SEQUENCE. IT SEEMED IMPOSSIBLE THAT THE APU WOULD CRANK FOR 5 MINS W/O LIGHTING OFF. I WAS ALSO HURRIED WITH WANTING TO GET THE UNIT RUNNING AND PWR SWITCHED OVER SO THAT THE #1 ENG COULD BE SHUT DOWN. AS I LOOKED BACK TO RESTART THE APU; I REMEMBER SEEING THE EGT GAUGE SHOWING AMBIENT TEMP; HOWEVER I DID NOT NOTICE WHETHER THE CRANK LIGHT WAS ON OR OFF BEFORE ACTIVATING THE START SWITCH AGAIN. ABOUT 5-10 SECS AFTER RESTARTING THE APU THE BRAKES WERE SET AND #2 ENG SHUT DOWN. ALTHOUGH ESSENTIAL WAS SELECTED TO #1 WE DID LOSE PWR FOR ABOUT 3 SECS. I BELIEVE THIS WAS CAUSED BY HIGH LOADS GENERATED FROM THE APU STARTER AND STILL HAVING 2 B PUMPS ON. THE CAPT FIRST SAW THE FIRE IN THE REFLECTION OF THE TERMINAL WINDOW; I LOOKED OUT AND SAW IT ALSO AT THE BASE OF THE RIGHT WING ROOT. KNOWING THAT THIS IS WHERE THE APU EXHAUST DUCT IS; I REACHED BACK AND TURNED THE APU OFF. THE FIRE WENT OUT. WITHIN SECS PEOPLE STARTED EXITING OUT THE OVERWING EMER EXITS. THE CAPT WENT AFT TO TRY AND STOP THEM. THE F/O CALLED GND FOR THE FIRE RESCUE EQUIP. I ELECTED TO THEN PULL THE APU FUEL SHUT OFF AND DISCHARGE THE FIRE BOTTLE. THE CAPT RETURNED TO THE COCKPIT AND THE SECURE CHKLIST COMPLETED. 10-20 PEOPLE HAD EVACUATED OVER THE LEFT WING. NO ONE WAS HURT. TOUR WINDOWS WERE DAMAGED IN THE VICINITY OF THE FIRE DUE TO WARPING. THE APU START SWITCH IS POORLY DESIGNED. IT SHOULD NOT BE ABLE TO RETURN ITSELF TO THE OFF POS AS IT FREQUENTLY DOES. IT SHOULD ALSO BE LOCKED OUT FROM THE START POS ONCE THE APU TURNING. AN APU TACHOMETER WOULD ALSO PROVIDE A BETTER INDICATOR OF APU STATUS. THE FREQ METER ONLY INDICATES APU RPM AT IDLE SPDS OR HIGHER. SUPPLEMENTAL INFO FROM ACN 92188: THE CONTROL OF THE PAX WAS DIFFICULT BECAUSE THE PAX ADDRESS SPEAKERS ARE UNDER THE OVERHEAD BAGGAGE BINS; AND THE PAX WERE STANDING IN THE ISLE. THE PAX ADDRESS VOL WAS ON HIGH SETTING BECAUSE IT GETS ITS AIR GND SENSE FROM THE #1 ENG LOW OIL PRESSURE LIGHT AND THE #1 ENG WAS STILL RUNNING. THE OVERWING EXITS WERE OPEN AND #1 ENG WAS RUNNING; MAKING IT DIFFICULT TO HEAR IN THE CENTER OF THE ACFT. SUPPLEMENTAL INFO FROM ACN 92073: SEVERAL SECS LATER 4 OR 5 PAX APPEARED ON THE RIGHT WING AND STARTED MILLING AROUND. BECAUSE THE FIRE WAS OUT I MADE THE ANNOUNCEMENT FOR THE PAX TO RETURN TO THEIR SEATS AS THE PROB WAS TAKEN CARE OF. I TOLD PEOPLE THE JETWAY WAS BEING PULLED INTO POS AND WE WOULD ALL EXIT NORMALLY VIA THE JETWAY. CALLED FIRE TRUCKS ON ATC CTL FREQ. PAX ON RIGHT WING RETURNED TO ACFT. FOUND OUT LATER SOME PAX DEPLANED VIA LEFT WING AND BELT LOADER THAT HAD PULLED UP TO LEFT WING. MOST PAX USED JETWAY. NO INJURIES.