MD80 CROSSED RWY AT DFW WITHOUT ATC CLRNC.

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WE WERE INSTRUCTED TO HOLD SHORT OF RWY 18L AT ALPHA. I ACKNOWLEDGED THAT CLRNC AND THE CAPT STOPPED THE ACFT AT TXWY ALPHA SHORT OF RWY 18L. I ASKED THE CAPT IF HE WANTED ME TO START THE APU AND HE SAID I COULD START IT ON THE TXWY ALPHA BRIDGE. UPON STOPPING ON TXWY ALPHA SHORT OF RWY 18L I HEARD THE TWR SAY; 'ACR X CLRED TO CROSS RWY 18L; STAY WITH ME ON 124.15 ON THE OTHER SIDE.' I RESPONDED; 'ACR X CLRED TO CROSS RWY 18L; 124.15 ON THE OTHER SIDE.' THERE WAS NO RESPONSE FROM THE TWR; NOR WAS ONE EXPECTED. THE CAPT LOOKED L TOWARD THE APCH END OF RWY 18L AND NOTED AN ACFT IN POS ON RWY 18L. WE CROSSED RWY 18L AT TXWY ALPHA. ABOUT HALFWAY ACROSS RWY 18L THE CAPT ACCELERATED THE ACFT AS HE NOTED THE ACFT ON THE APCH END OF RWY 18L APPEARED TO BE ROLLING AND ANOTHER ACFT WAS ENTERING RWY 18L AT THE APCH END. UPON CLRING RWY 18L WE SAW AN ACR Y ACFT CONTINUING ITS TKOF ROLL AND LIFTING ABOVE AND BEHIND US. THE ACR Y ACFT DID NOT COME CLOSE TO OUR ACFT. THERE WAS NO COMMUNICATION BY ANYONE ON THE TWR FREQUENCY DURING OUR RWY CROSSING. AFTER CLRING RWY 18L THE CAPT ASKED THE TWR IF ACR X WAS CLRED TO CROSS RWY 18L. THE TWR RESPONDED THAT ACR X WAS TOLD TO HOLD SHORT OF RWY 18L. THEN THE ACR Y FLT ASKED THE TWR IF AN ACFT HAD CROSSED THE RWY WHILE THEY WERE TAKING OFF. THE TWR ANSWERED THAT THEY WERE LISTENING TO THE TAPE.