user: Question: What was the engine class that was built on 1906-08?
Columns: ['Class', 'Firebox', 'Cylinder size', 'Valves', 'Valve gear', 'Number built', 'Years built']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['E2', 'Radial-stay', '20 ½” x 26”', 'Slide', 'Stephenson', '88', '1901-02']
SQL:

assistant: SELECT "Class" FROM "Table" WHERE "Years built" = '1906-08';