FLT CREW WAS TAXIING TO RWY 31C AT MDW; AND CROSSED THE HOLD LINE FOR 31L.

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OUR INITIAL TAXI CLRNC FROM SOUTH RAMP WAS VIA TXWY Y; TXWY K; TXWY F; HOLD SHORT RWY 31L. WE WERE UNABLE TO EXIT THE RAMP TO TXWY Y BUT F2 WAS CLR AND REQUESTED TO TAXI VIA F2; WHICH GND; AFTER SEEING OUR LOCATION; SAID THAT WOULD BE OK. TAXIING OUT; THE FO HAD THE CHART ON HIS SIDE AND I HAD A GARMIN SAFE TAXI DIAGRAM DISPLAYED. APCHING TXWY P; OFF TO MY L; I COULD SEE A RWY 31L HOLD LINE FOR TXWY P; WHILE IN FRONT OF ME A HOLDING POS SIGN FOR ILS CRITICAL AREA AND A RWY HOLD POS SIGN; WHICH MUST HAVE BEEN THE RWY 31L (WHAT I MUST HAVE THOUGHT WAS RWY 31C). VISUALLY; I WAS LOOKING AT 2 PARALLEL RWYS (31C; 31R) WHILE CONTINUING TO TAXI SLOW; I WAS LOOKING FOR THE RWY 31L HOLD WHEN I REALIZED THAT I HAD JUST CROSSED THE LINE. GND CALLED AND ADVISED THAT I HAD TAXIED PAST THE HOLD LINE. GND ASKED US TO CONTINUE TO TAXI BEHIND THE ACFT HOLDING FOR RWY 31C ON TXWY F. A CONTRIBUTING FACTOR WAS I LET THE FO SWITCH AND GET THE LATEST ATIS AT A CRITICAL TIME. ADDITIONALLY; I NEVER ACTUALLY CROSSED THE RWY BUT RATHER ENCROACHED THE SAFETY APCH AREA. I ALSO DON'T REMEMBER SEEING A MODIFIED TXWY CTRLINE FOR RWY 31L; IF IT WAS THERE I DON'T RECALL IT. I MUST HAVE CONFUSED THE RWY 31L/ILS AS RWY 31C; EVEN THOUGH ILS SIGNS DO NOT HAVE THE RWY IT IS ASSOCIATED WITH DISPLAYED. WITH BOTH THE ILS CRITICAL AREA HOLD SIGN (FOR RWY 31C) AND THE RWY 31L SIGNS TOGETHER WAS A CONTRIBUTING FACTOR. THE SIGNS MUST LOOK SOMETHING LIKE: '31L ILS.' A CLRNC TO TAXI TO HOLD SHORT OF THE RWY 31L APCH; WOULD HAVE BEEN HELPFUL; AS THIS WOULD BE ALERTING THE PLT THAT THEY WOULD NEVER CROSS THE RWY. AFTER ARRIVING AT MY DEST; I CALLED BACK TO THE TWR AND DISCUSSED THE MATTER BRIEFLY AND WAS TOLD THAT THEY HAVE A LOT OF PROBS IN THAT AREA. HAVING A SIGN '31L APCH' AND '31C ILS;' WOULD HAVE HELPED ME AND OTHERS IN THIS SITUATION. RENAMING THE RWY 31L SIGN TO RWY 31L APCH WOULD HAVE ALSO HELPED.