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

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'