user: Question: On what date was the number of locomotives 421-6?
Columns: ['Class', 'Wheels', 'Date', 'No. built', 'Loco Nos.']
Types: ['text', 'text', 'text', 'real', 'text']
Sample row: ['H1', '4-4-2', '1905-6', 5.0, '37-41']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "Loco Nos." = '421-6';