What the least quantity having a T2A class?
CREATE TABLE "first_conversion_period_1867_1887" (
  "class" text,
  "railway_number_s" text,
  "quantity_rebuilt" real,
  "rebuild_year_s" text,
  "type" text
);
SELECT MIN("quantity_rebuilt") FROM "first_conversion_period_1867_1887" WHERE "class"='t2a';