Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
On what date was the number of locomotives 421-6?

### Inputs:
table_name: table, columns: ['Class', 'Wheels', 'Date', 'No. built', 'Loco Nos.']

### Response:
SELECT Date FROM table WHERE Loco Nos. = 421-6