What are the fleet number for the 4-6-4t locomotive?
CREATE TABLE "belfast_and_county_down_railway" (
  "type" text,
  "fleet_numbers" text,
  "quantity_made" text,
  "manufacturer" text,
  "date_made" text
);
SELECT "fleet_numbers" FROM "belfast_and_county_down_railway" WHERE "type"='4-6-4t';