What year was the class 253 larger than 18 built?
CREATE TABLE "class_253_and_254" (
  "class" text,
  "operator" text,
  "number" real,
  "year_built" text,
  "cars_per_set" real,
  "unit_numbers" text
);
SELECT "year_built" FROM "class_253_and_254" WHERE "class"='class 253' AND "number">18;