user: Question: What year was the Class 253 larger than 18 built?
Columns: ['Class', 'Operator', 'Number', 'Year Built', 'Cars per Set', 'Unit Numbers.']
Types: ['text', 'text', 'real', 'text', 'real', 'text']
Sample row: ['Class 253', 'BR Western Region', 27.0, '1975-1977', 9.0, '253001-253027']
SQL:

assistant: SELECT "Year Built" FROM "Table" WHERE "Class" = 'Class 253' AND "Number" > 18.0;