B737-200 HAS TO CHANGE TKOF RWY TO RWY 33L FROM RWY 27 AT BOS. CTLR ISSUES TAXI CLRNC VIA TXWY C AND 'HOLD SHORT OF RWY 33L.' ACFT TAXIES ACROSS RWY 33L ON THE WAY TO TXWY C. CTLR EXPECTED ACFT TO HOLD SHORT OF RWY 33L 'ON THE WAY' TO TXWY C.

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WE TAXIED TO AND HELD SHORT OF RWY 27 FOR TKOF; WAITED FOR WT AND BAL INFO. WT AND BAL TOLD US WE WERE TOO HVY FOR RWY 27. WE TOLD TWR WE NEEDED RWY 33L. TWR TOLD US TO TAXI DOWN RWY 27; L ON TXWY C; HOLD SHORT OF RWY 33L. AS WE PULLED ONTO RWY 27; THE CAPT ASKED ME TO MAKE A PA INFORMING PAX OF EXTRA TAXIING. AS I COMPLETED PA OUR NOSE CROSSED ONTO RWY 33L AND TWR ASKED IF WE WERE HOLDING SHORT OF RWY 33L. CAPT INFORMED TWR; NO; WE THOUGHT WE WERE CLRED TO CROSS. TWR TOLD US WE HAD LNDG TFC FOR RWY 33L 2 OR 3 MI OUT. TFC WAS IN SIGHT. WE CONTINUED ACROSS RWY 33L WITH NO FURTHER INCIDENT. IF TWR CTLR DID NOT WANT US TO CROSS RWY 33L ON OUR WAY TO THE END FOR TKOF; BETTER INSTRUCTIONS WOULD HAVE BEEN; 'TURN R ON RWY 27; TAXI AND HOLD SHORT OF RWY 33L.' CONTRIBUTING FACTORS: 1) FIRST LEG TOGETHER FOR CAPT AND FO. 2) LATE WT AND BAL INFO REQUIRING CHANGE OF PLANS; CHANGE OF RWY. 3) BUSY TWR CTL FREQ. 4) UNUSUAL OPS; CHANGE RWYS; TAXI DOWN USUAL RWY. 5) PA WHILE ON ACTIVE RWY. 6) CAPT AND FO IN DIFFERENT LOOPS. CAPT ON TWR FREQ -- FO ON COMPANY FREQ. CAPT ON TWR FREQ -- FO ON PA. SUPPLEMENTAL INFO FROM ACN 371996: I WAS SURPRISED WHEN TWR ASKED ABOUT HOLDING SHORT. AFTERWARD; IN RETROSPECT; DO FEEL IT WOULD HAVE HELPED HAD I BEEN MORE PRUDENT AND CAUTIOUS IN CONFIRMING THAT WE ARE CLRED TO CROSS RWY 33L ENRTE.