What is the total amount of freight produced of u28c?
CREATE TABLE "general_electric" (
  "prr_class" text,
  "builder_s_model" text,
  "build_date" text,
  "total_produced" real,
  "wheel_arrangement" text,
  "service" text,
  "power_output" text
);
SELECT "total_produced" FROM "general_electric" WHERE "service"='freight' AND "builder_s_model"='u28c';