CREATE TABLE table_28015 ("Class" CLOB, "Firebox" CLOB, "Cylinder size" CLOB, "Valves" CLOB, "Valve gear" CLOB, "Number built" CLOB, "Years built" CLOB)

What was the engine class that was built on 1906-08?
SELECT "Class" FROM table_28015 WHERE "Years built" = '1906-08'
SELECT "Class" FROM "table_28015" WHERE "Years built" = '1906-08'