CAPT OF A B727 OVERSHOT ASSIGNED ALT DUE TO DISTR OF A ROUTING CONFLICTION. ATC INTERVENED IMMEDIATELY AFTER THE FLC'S RECOGNITION OF THE ALTDEV TO ALSO QUESTION THEIR ASSIGNED RTE WHICH WAS DIFFERENT THAN THE CREW'S PDC ROUTING.

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ZDV CALLED TO VERIFY OUR RTE OF FLT AFTER CKW VOR. THE CALL CAME JUST AS I WAS APCHING LEVELOFF; AND JUST AFTER I HAD TURNED OUTBOUND FROM CKW. A QUICK CHK WITH MY COPLT SHOWED THAT I HAD MISSED SEEING THE AMENDED RTE OF FLT ON THE PDC; ALTHOUGH SHE LATER SAID THAT SHE THOUGHT I WAS AWARE OF IT. SINCE WE WERE STILL BASICALLY OVER THE VOR; NO PROB. (ZDV ASSIGNED A HDG FOR TFC AND ASSIGNED A RERTE ANYWAY.) THE PROB THAT DID OCCUR WAS THAT THIS WHOLE SEQUENCE; EVEN THOUGH IT TOOK ONLY A FEW SECONDS; STARTED JUST PRIOR TO LEVELOFF. MY ATTN WAS MOMENTARILY DIRECTED TO RESOLVING THE ROUTING; AND AWAY FROM THE ALT. THE ALT WARNING SOUNDED AT 300 FT HIGH; AND I IMMEDIATELY RETURNED TO ASSIGNED ALT OF FL280; BUT MOMENTARILY REACHED 500 FT HIGH BEFORE THE CLB COULD BE STOPPED. ZDV NOTED THE ALT; BUT BY THEN WE HAD ALREADY RETURNED TO FL280. THERE WERE NO CONFLICTS; AND NO TFC AT ALL WITH RANGE OF TCASII (40 MI SCOPE). CONTRIBUTING FACTORS: 1) OUR COMPANY REGULARLY FILES A RTE THAT ATC NEVER APPROVES ON THE ORIGINAL CLRNC; BUT ALMOST ALWAYS APPROVES WHEN REQUESTED AT ALT. I HAD NOT FLOWN THIS FLT FOR A FEW MONTHS; FORGOT THE 'ROUTINE' AND WAS ABOUT TO FLY THE FILED RTE. THE MOMENTARY CONFUSION RESULTED IN A LACK OF ATTN TO LEVELOFF. 2) RTE AMENDMENTS ARE INDICATED ON THE PDC BY BEING ENCLOSED BTWN DASHES (-- CHANGE --). THIS IS VERY SUBTLE AND EASY TO MISS. I CANNOT UNDERSTAND THE NEED FOR CODE AND SYMBOLS WHEN PLAIN TEXT COULD BE USED. WHY NOT SIMPLY SAY; IN NICE BIG LETTERS THAT ARE HARD TO MISS 'AMENDED ROUTING'?