With a Type of 4-6-4t, what is the sum Withdrawn?
CREATE TABLE "hebburn_line_locomotives" (
  "locomotive" text,
  "type" text,
  "builder" text,
  "builder_s_no" text,
  "built" real,
  "entered_service" real,
  "withdrawn" real
);
SELECT SUM("withdrawn") FROM "hebburn_line_locomotives" WHERE "type"='4-6-4t';