MLG FLC TAXIES ONTO ACTIVE RWY WITHOUT CLRNC.

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FO LANDED ON RWY 35. SLOWED ENOUGH TO TURN OFF ON TXWY B. CAPT LOOKED L TO MAKE TURN AND SAW ARPT VEHICLE (FLASHING YELLOW CAB LIGHT) ON TXWY B. TAXIED TO NEXT INTXN (RWY 6/24) AND TURNED L ONTO IT. STOPPED. WHEN NO INSTRUCTIONS RECEIVED ON RADIO; THEN STARTED TO TAXI ONTO TXWY A. UPON CONTACTING GND CTL; WERE CLRED TO GATE AND ASKED IF SAW GREEN LIGHT. DIDN'T UNDERSTAND COMMENT SO SAID WOULD CALL FROM GATE. CALL TO TWR INDICATED CAPT HAD TAXIED ONTO AN ACTIVE RWY 6 WHICH WAS BEING USED BY LIGHT ACFT. ONE HAD TO GAR. CAPT EXPLAINED SIT AND ACTIONS. HUMAN PERFORMANCE: CAPT WAS LINE CHK AIRMAN DOING IOE OF NEW F2O. AFTER LNDG (BY FO) NO RADIO XMISSIONS OF ANY KIND WERE HEARD (IE; NO HOLD SHORT; NO TAXI THROUGH INTXN; NO CONVERSATIONS WITH OTHER ACFT; NO GAR; ETC). AFTER TURNING OFF RWY 35; THINK FO POSSIBLY MOVED RADIO SELECT SWITCH FROM TWR TO OTHER SIDE IN ATTEMPT TO CONTACT GND CTL PRIOR TO SELECTING FREQ (PROBABLY REASON FOR GREEN LIGHT). AFTER CLRING RWY 6/24 CONTACTED GND. CAPT COMMENTS: MHT USES ALL RWYS -- ALL AT SAME TIME; EG; LAND ON RWY 35; TKOF ON RWY 17; TKOF AND LAND ON RWY 6/24. ON APCH CTL AND TWR FREQ HEARD VOICE OF MORE THAN 1 CTLR GIVING CLRNCS; INSTRUCTIONS. (WERE THEY DOING TRAINING; TOO?)