A CL65 CREW TAXIED BEYOND THE HOLD SHORT LINE FOR MDT RWY 31 ON TXWY A CAUSING A RWY INCURSION.

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OUR FLT WAS CLRED TO TAXI TO RWY 31 AT MDT. OUR TAXI RTE FROM THE RAMP WAS TXWY E TO TXWY A TO RWY 31. AFTER TURNING ONTO TXWY A; THE FO AND I WERE COMPLETING THE BEFORE TKOF CHKLIST WHEN THE TWR INSTRUCTED US TO HOLD SHORT OF RWY 31. LOOKING OUT THE WINDOW; IT WAS APPARENT THAT THERE WAS NO HOLD SHORT LINE IN FRONT OF US. I INFORMED THE TWR THAT WE MAY HAVE PASSED THE HOLD SHORT LINE. THE HOLD SHORT LINE FOR RWY 31 IS IN A NONSTANDARD LOCATION APPROX 300 FT E OF THE TXWY F AND TXWY A INTXN. PRIOR TO TKOF; THE TWR ASKED ME TO CALL THEM UPON OUR ARR. WHEN I CALLED THE MDT TWR; THE CTLR WHO ANSWERED THE PHONE STATED THAT THEY HAVE HAD PROBS WITH RWY INCURSIONS AT THAT INTXN SINCE THEY'VE MOVED THE HOLD SHORT LINE. (THE CTLR SAID APPROX 17 INCURSIONS.) THE CTLR ALSO STATED THAT HE WOULD HAVE TO FILE A PLTDEV RPT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PLT STATED THAT BECAUSE IT WAS A NIGHTTIME OP AND THE CREW WAS COMPLETING A CHKLIST; HE DID NOT RECALL SEEING ANY TXWY MARKINGS. HE HAS SPOKEN WITH HIS ACR PRINCIPAL OPERATING INSPECTOR WHO DID NOT THINK THERE WOULD BE ENFORCEMENT ACTION TAKEN AS A RESULT OF THIS EVENT.