A B737-200 FLC WAS TOLD TO HOLD SHORT OF RWY 15R DURING TAXI TO RWY 24; BUT FAILED TO DO SO. A SMA WAS ABLE TO LAND ON THE RWY IN SPITE OF THE ENCROACHMENT.

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WE WERE TAXIING OUT TO RWY 24 FROM TERMINAL. OUR CLRNC WAS TAXI VIA TXWY E HOLD SHORT OF RWY 15R. I TURNED THE CORNER ON TXWY E AND SAW THE RWY 10/28 SIGN; BUT THERE WAS NO RWY 15R/33L SIGN. I TAXIED ONTO RWY 28; I REALIZED AT THE SAME TIME THE GND CTLR ADVISED 'HOLD POS' THAT I WAS ALSO ON RWY 15R. HOLDING WITH MY NOSE CLOSE TO OR JUST OVER THE EDGE OF RWY 15R; A BI-PLANE WAS ON SHORT FINAL FOR RWY 15R. HE LANDED AND WE WERE CLRED ACROSS. I WAS FLYING AN IOE TRIP. I WAS RECEIVING INSTRUCTION FROM ANOTHER CAPT AS I TRANSITIONED FROM B737-300 TO THE B737- 200. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS RPTR SAID THAT HE DID NOT SEE ANY SIGN FOR RWY 15R/33L AS HE ENTERED THE INTXN. HE SAID THAT THE VISIBILITY WAS GOOD AND THAT HE WAS LOOKING FOR THE SIGNS. HE HAS NOT OPERATED AT ISP VERY OFTEN. THIS ANALYST ASKED IF HE SAW A SIGN FOR RWY 33L AND THE RPTR SAID NO. HE ALSO STATED THAT HE DID REF THE COMMERCIAL ARPT PLAN VIEW DRAWING; BUT DID NOT REALIZE UNTIL TOO LATE THAT THE RWYS AND TXWY E INTERSECT AT THE SAME POINT.