user: Question: Tell me the sum of cars per set for operator of London Midland
Columns: ['Class', 'Operator', 'No. Built', 'Year Built', 'Cars per Set']
Types: ['text', 'text', 'real', 'text', 'real']
Sample row: ['Class 153', 'Arriva Trains Wales', 70.0, '1991–1992', 1.0]
SQL:

assistant: SELECT SUM("Cars per Set") FROM "Table" WHERE "Operator" = 'London Midland';