CRJ-100 FLT CREW HAS A RWY INCURSION AT DTW.

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WE LANDED ON RWY 4L IN DTW. ATC TAXI INSTRUCTIONS AFTER LNDG WERE; 'A7; S ON TXWY A; HOLD SHORT RWY 4R ON A5.' TAXI INSTRUCTIONS WERE UNDERSTOOD AND COMPLIED WITH. WE WERE HOLDING SHORT OF RWY 4R AT A5 FOR SEVERAL MINS. GND CTL FREQ WAS IN CTL OF THE FLT. THE GND FREQ WAS VERY CONGESTED. AFTER SEVERAL MINS OF HOLDING SHORT OR RWY 4R; I HEARD; 'ACR X TAXI S ON TXWY Y.' THE FO HEARD THE CLRNC ALSO AND REPLIED TO ATC; 'ACR X TAXI S ON TXWY Y.' I LOOKED TO THE APCH END OF RWY 4R AND SAW AN RJ THAT LOOKED TO BE HOLDING IN POS ON RWY 4. I APPLIED THRUST TO GET THE ACFT MOVING FORWARD. AT THE SAME TIME; I ASKED MY FO TO CLARIFY WITH ATC TO TAXI S OF TXWY Y AS THIS DIRECTION WAS AWAY FROM OUR INTENDED GATE. SIMULTANEOUSLY; I SAW THE RJ AT THE APCH END OF RWY 4R MOVING DOWN THE RWY AND I HEARD THE GND CTLR TELLING US TO STOP. THE ACFT DID PENETRATE THE HOLD SHORT LINE. IT DID NOT PENETRATE THE RWY EDGE. APPARENTLY THERE WAS ANOTHER XXX (SAME CALL SIGN NUMBER) ON THE SAME FREQ. I DID NOT HEAR A SPECIFIC CALL SIGN USED AND I ASSUMED THE 'XXX TAXI S ON TXWY Y' WAS FOR OUR FLT. I SHOULD HAVE IMMEDIATELY CLARIFIED WITH ATC THAT IT WAS 'XXX' THAT WAS CLRED TO CROSS RWY 4R AND TAXI S ON TXWY Y. THE RADIO CONGESTION ON THE FREQ WAS SUCH THAT ABBREVIATED XMISSIONS WERE BEING USED. UNFORTUNATELY; THIS ADDED TO THE INCIDENT. SUPPLEMENTAL INFO FROM ACN 698901: I CALLED GND AND ASKED WHAT DIRECTION HE WANTED US TO GO ON TXWY Y SINCE WE WERE PARKING. HE THEN CAME BACK AND SAID; 'XXX HOLD SHORT OF RWY 4.' THE ACFT WAS STOPPED BUT WAS PAST THE HOLD BAR; NOT PROTRUDING THROUGH THE RWY MARKINGS. I LATER TOLD THE CTLR HE SAID ;'XXX GO S ON TXWY Y.' HE THEN REPLIED THAT HE WAS SPEAKING TO ANOTHER COMPANY'S XXX; HOWEVER; NO COMPANY WAS NAMED IN THE XMISSION.