COMMUTER ACFT WAS CLRED FOR TKOF AND ROLLING WHILE AN ACR WAS XING RWY.

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FLYING INTO BOS ON JUN/FRI/92; CALL SIGN 'ACR X.' LANDED OUT OF VISUAL/ILS APCH ON RWY 27; CLRED RWY ON TAXIWAY X- RAY AND HELD BEHIND AN ACR LGT (BOTH OF US WAITING TO CROSS RWY 22R) WHICH WAS BOSTON'S DEP RWY AT THIS TIME. BOS IS THE ONLY UNITED STATES ARPT WHICH REQUIRED ATC READBACK OF HOLD SHORT INSTRUCTIONS. IT IS ALWAYS MENTIONED ON THE ATIS AND IN OUR COMPANY FLT OPS MANUAL. AFTER SITTING ABOUT 3 MINS; THE TWR CLRED 'ACR LGT TO CROSS 22R AND IN THE SAME SENTENCE CLRED ACR X TO CROSS 22R CONTACT GND POINT 9 ON THE OTHER SIDE.' BOTH ACFT READ BACK THE ATC INSTRUCTIONS. ACR LGT MOVED AND WE FOLLOWED. AFTER I CROSSED THE 22R HOLD LINE WITH THE NOSEWHEEL; ACFT MOVING SLOWLY; I REACHED UP AND TURNED ON THE WHITE WINGTIP STROBES WHICH HAS BEEN MY PERSONAL SAFETY HABIT WHEN XING AN ACTIVE RWY. I THEN HEARD THE TWR SAY 'ACFT Y CLRED FOR TKOF 22R.' I IMMEDIATELY ASKED THE FO 'DID YOU JUST HEAR THEM CLR SOMEBODY TO GO ON 22R?' I LOOKED TO THE R AND SAW AN SMT COMMUTER Y WITH LNDG LIGHTS ON; ON TKOF ROLL ON 22R. I IMMEDIATELY STOPPED THE AIRPLANE WITH THE NOSE MAYBE 10 FT ONTO THE 150 FT WIDE RWY. AT THE SAME TIME THE FO ASKED THE TWR 'CONFIRM ACR X CLR TO CROSS 22R?' THE TWR SAID 'NEGATIVE -- HOLD YOUR POS.' THE SMT Y ROTATED ABOUT 5 SECONDS LATER; APPROX 6000 FT AWAY AND THE TWR IMMEDIATELY CLRED US AGAIN TO CROSS 22R. I KNEW THAT WE HAD CLRNC TO CROSS; AND THAT THE FO HAD READ IT BACK TO THE TWR. I WAS SHOCKED THAT BOS TWR HAD CLRED THE COMMUTER TO TKOF BEFORE WE WERE ACROSS THE ACTIVE. I THINK IT WAS AN 'ERROR' ON THEIR PART AS I CAN'T BELIEVE THAT IT'S LEGAL TO HAVE AIRPLANES XING THE DEP END OF THE RWY WHILE OTHERS ARE ON TKOF. NOTHING ELSE WAS SAID ON RADIO. AFTER PARKING AT BOS; I WENT DOWN TO OPS AND CALLED THE TWR TO CLR ANY MISUNDERSTANDINGS. I TALKED TO A SUPVR WHO TOLD ME ANOTHER SUPVR WAS WORKING THE TWR POS AND THAT AS HE HADN'T HEARD ANYTHING ABOUT IT; IT WAS PROBABLY NO BIG DEAL. I THANKED HIM AND DECIDED TO FILE A NASA RPT JUST IN CASE SOMEBODY ELSE THINKS IT IS A BIG DEAL. I'M A 10 MONTH CAPT OF THIS ACFT (LGT); VERY CAREFUL AND CAUTIOUS AND AM SURPRISED BY THE BOS TWR'S CASUAL ATTITUDE ABOUT THIS INCIDENT.