RWY 31C INCURSION AS PIC OF A B737-500 CROSSES A SET OF CONFUSING RWY HOLD LINES AT MDW; IL.

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I AM A RESERVE CAPT AT ZZZ. I WAS ASSIGNED THE TRIP THE DAY PRIOR. I HAVE FLOWN THE SAME PAIRING WITHIN THE PRIOR 45 DAYS. I HAD NOT MET OR FLOWN WITH MY FO PRIOR TO THIS. WE ACCOMPLISHED A GOOD CREW BRIEF PRIOR TO PUSHBACK. WE PUSHED BACK ON TIME FROM GATE. ONCE CLR OF THE TERMINAL WE ACCOMPLISHED THE 'PRE TKOF CHKLIST.' COMPANY AHEAD WAS TO THE FAR L AND WE FILLED IN THE FAR R LANE. COMPANY WAS CLRED FOR TKOF AND DEPARTED. WE THEN PROCEEDED TO SHIFT L TO HOLD SHORT AND I CROSSED THE 'HOLD SHORT LINE' FOR RWY 31C. MY FO QUICKLY INFORMED ME AND HE MADE A RADIO CALL TO FESS UP AND ASK IF WE NEEDED TO MAKE A 180 DEG AND GET OUT OF THERE. THE TWR CTLR ACKNOWLEDGED US AND INSTANTLY GAVE ONE OF OUR ACFT ON A 1 1/2 MI FINAL INSTRUCTIONS TO GO AROUND. TWR THEN GAVE 2 OTHER ACFT INSTRUCTIONS. WERE ASKED TWR AGAIN IF WE NEEDED TO MOVE (WE DIDN'T WANT TO MOVE WITHOUT HIS APPROVAL AND DID NOT KNOW IF ANYONE WAS BEHIND US). TWR RESPONDED THAT HE HAD GIVEN US 'POS AND HOLD' AND 'NOW I HAD TO SEND ANOTHER ACFT AROUND.' HE SENT AN ACFT AROUND AND THEN GAVE US CLRNC FOR TKOF. WE ARE CERTAIN TWR NEVER GAVE US THE 'POS AND HOLD.' THE REMAINDER OF THE FLT WAS UNEVENTFUL. POSSIBLE FACTORS WERE THE MULTIPLE RWY HOLD SHORT LINES AND TAXI LINES. I BELIEVE I WAS LOOKING AT THE WRONG LINE; THOUGH IT LOOKS NOTHING LIKE A HOLD SHORT LINE. WE WERE STILL 3 SHIP LENGTHS AWAY FROM THE EDGE OF THE RWY. I WAS NOT HURRIED; BUT MY DECISION TO MOVE TO THE L SO AS TO BE IN A BETTER POS FOR AN EXPEDITIOUS TKOF WAS THE KEY FACTOR.