user: Question: This engine with a cylinder size of 20 ½” x 26” and a firebox of Radial-stay was built when?
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 "Years built" FROM "Table" WHERE "Cylinder size" = '20 ½” x 26”' AND "Firebox" = 'Radial-stay';