user: Question: What is the total amount of Freight produced of U28C?
Columns: ['PRR Class', 'Builder’s Model', 'Build date', 'Total produced', 'Wheel arrangement', 'Service', 'Power output']
Types: ['text', 'text', 'text', 'real', 'text', 'text', 'text']
Sample row: ['GS4', '44 Ton', '1948–1950', 46.0, 'B-B', 'Switcher', 'hp (kW)']
SQL:

assistant: SELECT SUM("Total produced") FROM "Table" WHERE "Service" = 'Freight' AND "Builder’s Model" = 'U28C';