ZBW LCL CTLR ISSUED INSTRUCTIONS TO AN ACR AFTER LNDG TO CROSS RWY 22L. A COMPANY ACR WITH A VERY SIMILAR CALL SIGN ON THE APCH END OF RWY 22L ACKNOWLEDGED THE CLRNC AND CROSSED BEHIND A DEPARTING DC9.

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WE WERE IN A LONG TAXI LINE AT BOS WAITING FOR DEP. ATC WAS USING RWYS 22R&L FOR DEPS; AND MY CAPT AND I DISCUSSED HOW WE HADN'T SEEN RWY 22L USED FOR DEPS. THERE WAS A CONSTANT RATE OF DEPS; ONE AFTER ANOTHER. ACR Z; A DC9; WAS AHEAD OF US AND IT WAS CLRED INTO POS AND HOLD ON RWY 22R. WE HEARD OUR FLT 'ACR AABA CROSS RWY 22R; HEAD FOR ECHO.' MY CAPT AND I DISCUSSED QUICKLY THAT MAYBE WE WERE GOING TO DEPART OFF RWY 22L SINCE OUR COMPANY HAD FILED A MORE NORTHERN FLT PLAN THAN USUAL. I SAID I WAS GOING TO VERIFY THAT THE DC9 IS POS AND HOLD WAS GOING TO BE AT IDLE PWR SINCE WE WERE GOING TO BE PASSING RIGHT BEHIND IT. WE ASKED THE TWR 'IS THE DC9 GOING TO BE AT IDLE PWR FOR ACR AABA?' TWR RESPONDED THAT THEY DIDN'T UNDERSTAND AND TO CROSS RWY 22R AND HEAD FOR ECHO. ONCE AGAIN I ASKED; THIS TIME MORE SIMPLY; 'IS THE ACR Z DC9 TAKING OFF?' TWR RESPONDED NO; AND WE BEGAN TO CROSS BEHIND THE DC9. ABOUT 1/2 WAY ACROSS; I SAW THE DC9 BEGIN TO ADVANCE ON THE PWR AND I TOLD THE CAPT TO GET ACROSS QUICKLY. WE AVOIDED MOST OF THE JETBLAST AND CONTACTED GND AS INSTRUCTED. WHEN WE CHKED IN ON THE GND FREQ; WE WERE INSTRUCTED TO GO BACK TO TWR. I TOLD THEM THAT WE WERE JUST HANDED OFF TO GND AND THAT IS WHEN GND TOLD US WE HAD CROSSED USING ANOTHER COMPANY FLT (ACR AABC) CLRNC. WE WENT BACK TO TWR AND DEPARTED NORMALLY. THE OTHER FLT (ACR AABC) WAS INSTRUCTED TO MONITOR GND SO WE NEVER KNEW THEY WERE DOWNFIELD. WE DID NOT HEAR THEM READ BACK THE CLRNC TO CROSS AND WE ASKED THE TWR MULTIPLE TIMES ABOUT OUR CONCERN TO PASS BEHIND THE DC9 IN POS. IT HAPPENED THAT WE WERE #1. THEY (ATC) HAD BEEN USING RWY 22L; AND A COMPANY FLT WITH A VERY SIMILAR CALL SIGN WAS #1 TO CROSS DOWNFIELD. THE ONLY THING THAT MIGHT HAVE HELPED WOULD HAVE BEEN TO BE NOTIFIED OF A SIMILAR FLT NUMBER WOULD BE XING DOWNFIELD; OTHERWISE WE HAD NO IDEA ABOUT THEM. SUPPLEMENTAL INFO FROM ACN 488312: WHAT WE DIDN'T KNOW IS THAT OUR COMPANY WITH ONLY 1 DIGIT DIFFERENCE IN CALL SIGNS WAS RUNNING LATE AND HAD JUST LANDED ON RWY 27 AND WAS GOING TO CROSS RWY 22R TO GET TO THE GATE AREA. TWR'S INTENTION WAS TO GET OUR COMPANY ACROSS RWY 22R AND WE BOTH ENDED UP XING THE RWY EVEN THOUGH WE VERIFIED THE INSTRUCTIONS WITH THE CORRECT CALL SIGN. THE TWR FREQ WAS BUSY AND IT'S HARD TO TELL JUST WHERE THE MISCOM STARTED. EVERYTHING TURNED OUT FINE FOR ALL INVOLVED; BUT IT JUST REINFORCED TO MY CREW AND I HOW VIGILANT WE MUST ALWAYS BE AND TO ALWAYS HAVE SITUATIONAL AWARENESS (IE; IF IT DOESN'T SEEM RIGHT; IT PROBABLY ISN'T). THE BEST FIX WE COULD COME UP WITH FROM OUR STANDPOINT IS TO BE MORE DESCRIPTIVE IN OUR LOCATION AND SIT (IE; WE WANT TO VERIFY THAT YOU WANT US TO CROSS THE APCH END OF RWY 22R BEHIND THE DC9 IN POS AND HOLD AND CONTINUE ON TO RWY 22L).