B737-300 LAND RWY 36 AND ACCOUNT NO GATE ADVISED TO HOLD IN HOLDING AREA. FOLLOWED ACFT ACROSS RWYS 15/33 WITHOUT CLRNC. WAS RECLRED BACK TO HOLDING AREA TO THE W OF RWY 36. NO CONFLICT. RPTR SUGGESTS BETTER MARKING OF HOLDING AREA ON ARPT DIAGRAM.

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AFTER LNDG NORMALLY ON RWY 36 AT DCA ON TXWY S; WE CONTACTED GND CTL. HE ASKED IF OUR GATE WAS OPEN; WE SAID NEGATIVE. WE WERE BEHIND A COMPANY ACFT WHICH WAS XING RWY 15/33 AT TXWY S. GND CTLR INSTRUCTED US TO 'JUST TAXI INTO THE HOLDING AREA ON YOUR R; THERE.' WE BOTH UNDERSTOOD THAT TO MEAN CROSS THE RWY BEHIND THE COMPANY ACFT AND PROCEED TO THE PAD AREA ON THE N END OF TXWY L. AFTER XING THE RWY (AND VISUALLY CONFIRMING NO TFC; GND OR AIR; IN BOTH DIRECTIONS) WE TAXIED NW ON TXWY L AND NOTICED THERE WAS REALLY NO PLACE I COULD PARK WITHOUT BLOCKING ACCESS TO RWY 15 IF IT SHOULD BE NEEDED. I THEN MADE A L 180 DEG TURN AND ASKED FOR ALTERNATE INSTRUCTIONS. AT THAT POINT WE WERE TOLD TO TURN L AND CROSS RWY 15 AT TXWY S AND 'HOLD IN THE BLOCK.' THIS TIME IT WAS CLR HE MEANT THE HOLDING AREA AT N END OF TXWY K. AFTER XING RWY 15; IT OCCURRED TO ME THAT WE HAD INADVERTENTLY CROSSED A RWY WITHOUT A SPECIFIC CLRNC TO DO SO. EVEN THOUGH SAFETY WAS NOT COMPROMISED IN THIS SIT; IT SEEMS TO BE A POSSIBILITY DURING THE ARPT'S BUSY TIMES. I FEEL CERTAIN THAT WE WOULD HAVE QUESTIONED OUR INSTRUCTIONS; HOWEVER; IF THERE HAD NOT BEEN ANOTHER ACFT XING THE RWY IMMEDIATELY AHEAD OF US. ALSO; WE DID NOT QUESTION THE CTLR'S INSTRUCTION BECAUSE IT SEEMED CLR TO BOTH OF US AT THE TIME. THE LOCATION APPARENTLY MEANT BY THE CTLR WAS MORE PROPERLY BEHIND US AND NOT TO OUR R. IT SEEMS A CASE IN WHICH AMBIGUOUS INSTRUCTIONS COULD HAVE LED TO A MORE SERIOUS SIT. BLOCKS SHOULD BE CLRLY IDENTED ON OUR TAXI CHARTS.