ACR X HAD TO ABORT TKOF DUE TO ACR Y TAXIING ACROSS THE RWY. PLT DEVIATION.

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OUR FLT (ACR X) WAS CLRED 'INTO POS TO HOLD' ON RWY 9L AT PHL. AFTER ABOUT 30 SECS OR LESS IN POS WE WERE 'CLRED FOR TKOF.' WE HAD HELD WITH OUR NOSE TAXI LIGHT AND NORMAL NAV LIGHTS ON. I OFFERED THE THROTTLES TO MY F/O AND SWITCHED ON OUR LNDG LIGHTS. DURING THE TKOF ROLL; AT ABOUT 65-70 KTS; ACR Y WAS OBSERVED XING THE RWY AT TXWY C. WE ABORTED OUR TKOF AND SLOWED TO TAXI SPD WITH ABOUT 1500-2000' TO SPARE. AS WE CLRED THE RWY THE TWR ADVISED THAT ACR Y HAD XED W/O PERMISSION. IT WAS NIGHT WITH HIGH OVCST. SHOWERS WERE GENERALLY IN THE AREA; THE ATMOSPHERE WAS A BIT HAZY AND THE PAVEMENT WAS WET. PAVEMENT PAINTED MARKINGS WERE OBSCURED BY THE WET CONDITIONS. THE TKOF RWY HAD CHANGED FROM 27L TO 9L SUBSEQUENT TO OUR RECEIVING OUR CLRNC. ANY OR ALL OF THIS MAY HAVE CONTRIBUTED TO THIS SITUATION. THE ACTIONS OF ACR Y WOULD HAVE BEEN CONSISTANT WITH HEARING WHAT HE MAY HAVE EXPECTED; 'CLRED TO 27L;' W/O INSTRUCTIONS TO HOLD SHORT; THE PATH TO 27L XES 9L AT TXWY C; (A VERY SHORT DISTANCE FROM ACR Y'S GATE). THE PATH TO RWY 9L XES NO RWYS AND SO NO 'HOLD SHORT' WOULD BE APPROPRIATE. WITH THE RECENT RWY CHANGE; A MIND SET TO RWY 27L MAY WELL HAVE BEEN ESTABLISHED AND THE RWY 9L INSTRUCTIONS MISHEARD. FROM MY VIEWPOINT IN THE ACFT ON TKOF ROLL; THINGS LIKE THIS ARE ONLY IN THE SIMULATOR. MECHANICAL FAILURES ARE EXPECTED; BUT A RWY INCURSION IS SO RARE AND SO UNTHINKABLE THAT WHEN IT HAPPENS IT IS INCREDULOUS. THE PRESS OF HIGH DENSITY AIR TFC ENCOURAGES TWR CTLRS TO CALL RWY SEP AS CLOSELY AS POSSIBLE. THE TWR PERSPECTIVE WAS PRESUMABLY BETTER THAN MINE; ESPECIALLY WITH LESS THAN OPTIMUM VISUAL CONDITIONS. THUS; FOR A FEW MILLISECS I WAS INCLINED TO WANT ACR Y TO BE CLR IN TIME; EVEN IF NOT RIGHT; TO SQUEAK BY ON ANOTHER MARGINAL JUDGEMENT CALL BY A HARRIED CTLR. I WAS MOMENTARILY CHKING ENG INSTRUMENTS WHEN THIS OCCURRED. THE VERY ALERT AND COMPETENT ACTIONS OF MY F/O CONFINED THIS TO NO MORE THAN A LOT OF DISCUSSION. THE POTENTIAL OF THIS SITUATION REMINDS US ONCE AGAIN: THE IMPOSSIBLE CAN HAPPEN--YOU CAN NEVER BE TOO CAUTIOUS WHEN XING A RWY--AND WE MUST BE ALERT TO HEAR WHAT IS SAID AND AWARE OF HOW EASY IT IS; FOR WHAT YOU EXPECT TO HEAR TO BE WRONG.