B737-300 CAPT RPTS TXWY/RAMP MARKINGS AND LIGHTING OF HARD SURFACE 'ISLAND' BETWEEN TXWYS K AND D AT SFO ARE DIFFICULT TO SEE AND MAY BE INOPERATIVE OR TOO DIM.

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I LANDED RWY 28L AND TAXIED CLEAR ON TXWY T AND TRANSITIONED TO TXWY D; HEADED FOR THE GATE. THE TXWY BORDER LIGHTS (BLUE) THAT NORMALLY OUTLINE THE 'ISLAND' THAT EXTENDS E AND W BETWEEN TXWY K AND TXWY D WERE EITHER NOT ILLUMINATED OR WERE TURNED DOWN SO LOW AS TO BE INVISIBLE IN DARKNESS. THE TXWY CTRLINE LIGHTS THAT LEAD INTO TXWY D FROM THE N WERE ALSO ONLY PARTIALLY ILLUMINATED; AND WENT DARK AS THE TXWY JOINED TXWY A AND THE RAMP. AS I TAXIED S ON TXWY D AND TURNED SLIGHTLY RIGHT (W) TO JOIN THE RAMP; I REALIZED THAT I HAD ACTUALLY ENTERED THE EASTERN END OF THE 'ISLAND.' THIS AREA IS ENTIRELY PAVED. MOST OF THE AREA IS PAINTED GREEN; EXCEPT FOR THE AREA THAT I TAXIED ON; WHICH IS UNPAINTED CONCRETE THAT APPEARS EXACTLY AS THE TXWYS. AN ADDITIONAL VISUAL CLUE THAT SFO HAS RUNNING THROUGH THE 'ISLANDS' THAT ARE PAVED IS A LINE OF VERTICAL PLASTIC 'PICKETS.' THIS UNLIGHTED UNPAINTED AREA WAS ALSO DEVOID OF 'PICKETS' ON THE EASTERN END (WHICH I TAXIED ACROSS). WE SMOOTHLY TRAVERSED THE EASTERN END OF THE 'ISLAND.' THERE WERE NO OBSTACLES OR FOREIGN OBJECTS IN THIS AREA THAT MIGHT HAVE PRESENTED A HAZARD TO THE ACFT; AND A THOROUGH POST FLT INSPECTION WAS COMPLETED (BY ME). UNDER SIMILAR NIGHT CONDITIONS THAT MIGHT INCLUDE A RESTRICTION TO VISIBILITY (RAIN ON THE PAVEMENT); I'M SURE THAT I WOULDN'T HAVE EVEN REALIZED THAT I HAD TRAVERSED THE 'ISLAND.'