What was the model for PRR class of gf28a freight?
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 "builder_s_model" FROM "general_electric" WHERE "service"='freight' AND "prr_class"='gf28a';