user: Question: What is the average fleet size for the Shunter type introduced in 1959?
Columns: ['Class', 'Type', 'Introduced', 'Fleet Size', 'Numbers']
Types: ['text', 'text', 'text', 'real', 'text']
Sample row: ['Class 08', 'Shunter', '1953', 2.0, '08925, 08934']
SQL:

assistant: SELECT AVG("Fleet Size") FROM "Table" WHERE "Type" = 'Shunter' AND "Introduced" = '1959';