user: Question: Which year has a railway class of 250?
Columns: ['Gauge', 'Railway', 'Class', 'Works no.', 'Year', 'Builder']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['3 ft 6 in', 'Rhodesia Railways', '15th', '6936–6939', '1940', 'Beyer, Peacock']
SQL:

assistant: SELECT "Year" FROM "Table" WHERE "Class" = '250';