A B737-300 FLC PERFORMS AN UNAUTH INTXN TKOF AT RNO; NV.

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RNO GND CTL CLRED US TO TAXI TO RWY 16R. WHILE TAXIING N ON TXWY B THE TWR CLRED US FOR TKOF JUST PRIOR TO TXWY D. TXWY D INTERSECTS RWY 16R AT THE DISPLACED THRESHOLD MARKING. I MISTAKENLY ASSUMED THIS WAS THE DEP END OF RWY 16R AND BEGAN THE TKOF ROLL. DURING THE TKOF ROLL THE TWR ASKED IF WE HAD INTENDED TO DEPART FROM INTXN D. THE FO REPLIED 'YES.' SHORTLY AFTER TKOF I REALIZED THE MISTAKE I HAD MADE. I CALLED BACK TO THE TWR AND ASKED IF WE NEEDED CLRNC TO DEPART FROM INTXN D. THE TWR REPLIED 'YES YOU NEEDED TO COMMUNICATE THAT IF YOU WANTED TO DEPART FROM INTXN D.' I REPLIED THAT WE WERE SORRY AND APOLOGIZED FOR NOT COMMUNICATING. THE TWR REPLIED THAT 'IT'S A TEAM EFFORT; THERE WERE NO OTHER ACFT THAT COULD HAVE CAUSED A CONFLICT AND NOT TO WORRY.' AFTER LNDG I CALLED THE RNO TWR ON THE PHONE TO APOLOGIZE AGAIN. THE SUPVR REPLIED THAT 'IT WAS NOT A BIG DEAL; WE JUST EXPECTED YOU TO GO TO THE END OF THE RWY; SINCE YOU WERE CLRED FOR TKOF YOU 'OWNED' THE RWY. THERE WERE NO OTHER ACFT IN THE AREA SO THERE IS NO PROB.' I BELIEVE THIS MISTAKE WAS CAUSED BY GND FEATURES. FIRST; I NORMALLY TAXI OUT ON TXWY A; BUT FOR SOME REASON I USED TXWY B; SO I WAS SOMEWHAT UNFAMILIAR WITH THIS RTE. SECOND; WE WERE THE ONLY ACFT ON FREQ SO IT WAS VERY QUIET. THIRD; THE TWR CLRED US FOR TKOF WAY EARLY JUST PRIOR TO TXWY D. RWY 16R AT INTXN D IS THE DISPLACED THRESHOLD AND LOOKS LIKE A DEP END WITH AN OVERRUN. THE RWY AND LIGHTS EVEN ARE MOUNTED IN THE PAVEMENT HERE TO FURTHER CREATE THE ILLUSION OF A RWY END. FOURTH; TXWY B N OF D LOOKS NARROW AND UNMAINTAINED SO I WAS HESITANT TO CONTINUE PAST D. THE DISTANCE FROM D TO THE DEP END IS ONLY APPROX 1000 FT. UNFORTUNATELY; I CONTINUED TO TAXI AND TAKE OFF EVEN THOUGH I WAS NOT 100% SURE WHAT I WAS DOING WAS CORRECT; BUT I TALKED MYSELF INTO BELIEVING I WAS CORRECT; AND DID NOT ASK FOR CONFIRMATION BECAUSE EVERYTHING SEEMED RIGHT. IN THE FUTURE; I WILL QUESTION WHEN I AM NOT 100% SURE THAT WHAT I AM DOING IS EXACTLY CORRECT. SUPPLEMENTAL INFO FROM ACN 485574: I WAS XCHKING OUR TAXI RTE WHILE COMPLETING THE CHKLIST. I COULD SEE THAT TXWY B CONTINUED STRAIGHT AHEAD AND THAT MARKINGS DENOTING THE DISPLACED THRESHOLD FOR RWY 16R BEGAN THERE AT INTXN D. (TURNING THERE AT INTXN D DIDN'T SEEM QUITE RIGHT; BUT I COULDN'T QUITE DECIDE WHY.) I FAILED TO COMMUNICATE THIS MINOR SENSE OF UNEASE TO THE CAPT. AS WE STARTED OUR TKOF ROLL; TWR ASKED US IF WE WERE AWARE THAT WE WERE TAKING OFF FROM INTXN D AND ASKED IF THAT WAS WHAT WE WANTED TO DO. I FAILED TO SPEAK UP BECAUSE I WASN'T EXACTLY SURE WHAT WE WERE DOING WRONG. IN THE FUTURE; I WILL SPEAK UP WHEN I'M NOT SURE IF WHAT WE ARE DOING IS EXACTLY RIGHT.