ACR HVT FOLLOWED WRONG TAXI ROUTE NEARLY ENTERED WRONG RWY FOR TKOF AT ORD.

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WE WERE CLRED FROM THE C CONCOURSE AT ORD 'VIA THE INNER THE STUB THE PARALLELS TO T-1' FOR INTXN TKOF ON RWY 32L; AN UNUSUALLY COMPLICATED RTE NECESSITATED BY HVY TFC; AND ALSO TAXIING OUT WE HAD SOME MINOR ILLUMINATION PROBS IN THE COCKPIT. LATER; I LEARNED THAT I HAD TURNED TO THE RIGHT AT THE STUB AND WAS WAITING AT THE N-S RATHER THAN T-1; WHICH IN THE DARK; APART FROM ITS DIRECTION; IS SUPERFICIALLY AT RWY 27L SIMILAR. I WAS DOUBTFUL SINCE I ALWAYS USE A HDG BUG AS A RWY REF; AND THE COPLT WAS ALSO SOMEWHAT DOUBTFUL. WE WERE ABOUT TO DISCUSS IT FURTHER WHEN THE TWR ABRUPTLY CLRED US INTO POS. WE QUERIED IT AND IT WAS CONFIRMED AND SO I STARTED TO MOVE TOWARD THE RWY AND TURNED MY LIGHTS ON; AT WHICH POINT THE COPLT CALLED 'HOLD IT;' AND AN MLG COMPLETED ITS LNDG ROLL IN FRONT OF US. WE QUERIED THE TWR AGAIN; WHO SAID; 'LET'S CONFIRM WHERE YOU ARE.' AND ANOTHER VOICE ASKED; 'DID YOU PENETRATE THE RWY?'; TO WHICH WE COULD SAFELY REPLY 'NO.' IT WAS OF COURSE TOTALLY MY RESPONSIBILITY TO HOLD FAST UNTIL I WAS CERTAIN WHAT WAS GOING ON; DESPITE THE 'ARE WE CLRED ONTO THIS RWY?' 'YES.' INTERCHANGE. STILL; I WOULD SUGGEST THAT POINTS USED FOR INTXN TKOFS BE CLEARLY MARKED. AND SECONDLY; IF POSSIBLE; THE GND CTLR BE AWARE OF HIS MOVEMENTS. AN OUTSTANDING COPLT PREVENTED WHAT MIGHT WELL HAVE BEEN A VERY NASTY OUTCOME.