RWY INCURSION WHEN THE PIC OF A TAXIING B737 CROSSES OVER WHAT HE CONSIDERED LATER TO BE MISPLACED HOLD SHORT LINES NEAR RWY 31R AT MDW.

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TAXIED TO RWY 31C VIA TXWYS Y; P; G. COMPANY ACFT WAS HOLDING SHORT OF RWY 31C ON E1. APCHING COMPANY ACFT; CAPT TAXIED AROUND ACFT TO HOLD SHORT NEXT TO COMPANY ACFT ON E3. AS WE WERE DOING SO; WE WERE TOLD TO CLR RWY 31R BY TWR. WE REALIZED WE HAD CROSSED THE HOLD SHORT LINE FOR RWY 31R. WE CLRED THE HOLD SHORT LINE AND HELD SHORT OF RWY 31R ON E2. MDW 10-9B PAGE IS INCORRECT. THE HOLD SHORT LINES FOR RWYS 31C AND 31R ARE NOT PARALLEL. THEY ACTUALLY CONVERGE NEAR E3. THERE IS NO ROOM TO HOLD SHORT OF RWY 31C ON E3 AND REMAIN CLR OF RWY 31R. RECOMMEND CHANGING 10-9B PAGE TO REFLECT ACTUAL RWY MARKINGS. SUPPLEMENTAL INFO FROM ACN 617821: WE SKIRTED THE COMPANY ACFT AHEAD BY XING THE RWYS 13L/31R HOLD SHORT LINE IN THE VICINITY OF THE RWY 31R ILS HOLD SHORT LINE AS DEPICTED ON MDW 10-9B PAGE WHERE IT INTERSECTS THE TXWY E1. I TAXIED AROUND OUR OTHER ACFT IN THE OFF CHANCE HE WAS DELAYED; WE WOULD BE IN POS FOR AN IMMEDIATE TKOF. WE WERE AWARE OF AN ACFT ON FINAL TO RWY 31C WHICH WAS CIRCLING TO RWY 31R; IN VICINITY OF RUNTS; AN ACFT ACTUALLY OVER THE THRESHOLD TO RWY 31C OR TOUCHING DOWN AT THAT TIME. WITH THE LOWER CEILINGS; I MISTAKENLY ASSUMED RWY 31R NOT BEING IN USE; STAYING CLR OF THE ACTUAL RWY 31R. COMING BACK THROUGH MDW LATER; I TALKED WITH A SUPVR IN THE TWR. HE SAID NO HARM; NO FOUL; TRAINING WAS IN PROGRESS. HE ALSO MENTIONED SOME LINES HAD BEEN MOVED AROUND AND THEN MENTIONED SPACING; SIZE AND NUMBER OF ACFT THAT WERE ABLE TO BE ACCOMMODATED IN THAT AREA. THERE WERE 2 OTHER ACFT BESIDES OURSELVES WHO HAD TRANSGRESSED. RECOMMEND NOTE IN 10-7 PAGE AND EXCERPTS IN ASAP OCCURRENCE LETTER HOT SPOT?