B737 CAPTAIN REPORTS DEPARTING DESIGNATED TXWY AT EWR AFTER ATTEMPTED LEFT TURN FROM TXWY B TO TXWY DL. NIGHT RAINY CONDITIONS PREVENT SEEING TXWY EDGE MARKERS.

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AFTER A RETURN TO GATE FOR PASSENGER MEDICAL SITUATION; WE WERE TAXIING BACK OUT FOR DEPARTURE. WEATHER IN THE AREA HAD US WAITING ON THE TAXIWAYS FOR ABOUT 1 HOUR AND 10 MINUTES ALREADY. WHILE WAITING ON TXWY B SHORT OF TXWY G BEHIND A HEAVY JET; GROUND ASKED IF WE COULD MAKE THE LEFT TO TAKE TXWY A. I SAW THE SIGN FOR TXWY DL AND SAID YES WE COULD. AS I WAS MAKING THE TURN I REALIZED MARKERS INDICATING THE REGION BETWEEN TXWYS DL AND RE WERE IN FRONT OF US; BUT WE WERE UNABLE AT THAT POINT TO MAKE A TURN OUT OF THE AREA. WE NOTIFIED GROUND AND AN AIRPORT VEHICLE CAME OUT; REMOVED THE MARKER; AND WE WERE THEN ABLE TO TAXI THROUGH TO TXWY A AND PROCEED.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE ENTRANCE TO TXWY DL FROM TXWY B IS MARKED ON THE NORTH AND SOUTH SIDES BY SIGNS. THE CONFUSION OCCURRED WHEN IT WAS ASSUMED THAT THE SIGN IN VIEW WAS ON THE SOUTH SIDE AND THE HEAVY JET WAS BLOCKING VIEW OF THE NORTH SIGN. AS THE TURN WAS COMMENCED THE TXWY EDGE MARKER CAME INTO VIEW AND BLOCKED THE TURN. THE MARKERS DO NOT APPEAR TO BE VERY REFLECTIVE AND COULD NOT BE SEEN BEFORE THE TURN WAS COMMENCED.