CAPT OF A B727 INADVERTENTLY FAILED TO HOLD SHORT OF AN ACTIVE PARALLEL RWY DURING TAXI IN DUE TO UNFAMILIARITY WITH THE ARPT LAYOUT.

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AFTER LNDG ON EWR RWY 4R; I SLOWED THE ACFT TO EXIT L TOWARD THE TERMINAL. AS I PREPARED TO EXIT ON HIGH SPD TXWY L; TWR TOLD US TO CONTINUE DOWN THE RWY; CROSS RWY 29 AND TURN L AT TXWY Z. WE DID NOT RECEIVE A HOLD SHORT INSTRUCTION. THIS WAS AN UNEXPECTED CHANGE TO OUR BRIEFED EXIT PLAN. THERE WAS AN ACFT POSITIONING AND HOLDING FOR TKOF ON RWY 29 AS WE TAXIED ACROSS THAT RWY; A DISTR THAT CAPTURED A FEW MOMENTS OF OUR ATTN. WE EXITED THE RWY AND STARTED SLOWLY MOVING ALONG TXWY Z. I NOTICED A LARGE RAMP AREA AHEAD OF THE ACFT AND TO THE R; AND THE FO SWITCHED TO GND CTL. GND CTL TOLD US TO SWITCH BACK TO TWR. ABOUT THAT TIME I NOTICED A HOLD SHORT LINE WELL BEHIND ME. I AM UNSURE HOW FAR I WAS OVER THE LINE; BUT I WAS FAR ENOUGH TO MAKE ME VERY UNCOMFORTABLE. I LOOKED L AND CONFIRMED THAT RWY 4L EXTENDED TO TXWY Z. I SAW AN ACFT WELL DOWN THE RWY WITH ITS LNDG LIGHTS ON (INDICATING IT WAS CLRED FOR TKOF AND WOULD BE STARTING ITS TKOF ROLL; OR HAD ALREADY DONE SO). I REACTED BY PROCEEDING ACROSS THE RWY TO CLR THE POTENTIAL DANGER. THIS WAS AN UNFAMILIAR ARPT. TO HELP PREVENT SIMILAR INCIDENTS; I PERCEIVED 3 FACILITATING INDICATORS THAT RWY 4L MAY NOT EXTEND TO TXWY Z: 1) THE TWR CTLR INSTRUCTIONS WERE CONSISTENT WITH WHAT I EXPECT TO HEAR WHEN THERE ARE NO RWYS TO CROSS AFTER CLRING THE ACTIVE RWY. 2) THERE IS A LARGE RAMP AREA BEYOND THE RWY 4L DEP THRESHOLD; WHICH IS EASILY VISIBLE AS YOU PROCEED W FROM RWY 4R TOWARD RWY 4L ALONG TXWY Z. IT IS THE SAME COLOR PAVEMENT AS THE TXWY AND RWY. THE COMMERCIAL ARPT FAMILIARIZATION OVERVIEW PAGE SHOWS RWY 4L NOT EXTENDING TO TXWY Z.