WASHINGTON (AP) _ Transit officials released a preliminary report into last month's fire on the Washington Metro Monday, in which they blamed an unusual design along the stretch of track where the fire broke out.

``The factors that caused this particular fire are unlikely to recur in our system,'' said Fred Goodine, Metro Safety Officer.

The report said the cable to the electrified rail is embedded in a concrete slab running under the tracks. Two pieces of metal on the ends of the slab cut into the casing and insulation surrounding the wire, grounding the cable and causing the April 20 fire in a subway tunnel under the nation's capital.

The grounded cable then opened high voltage fuses, which shut off power and stranded a Virginia-bound train carrying 273 passengers. It took over 2 1/2 hours to evacuate them.

According to Metro, the scene of the fire _ between the Farragut West and Foggy Bottom stations _ is the only place on the subway system that has this kind of cable installation, which does not meet Metro's current standards. Work is underway to replace the cable.

District of Columbia firefighters, whose radios did not work underground, will use World War II era field phones if they have to fight another blaze inside the tunnels. Acting Fire Chief Thomas Tippett said Monday the field phones and spools of cable can be hooked up quickly.

Metro said it would also provide two-way radios to firefighters, should there be another emergency.