Table schema:
CREATE TABLE table_name_19 (date INTEGER, works_number VARCHAR, builder VARCHAR, number VARCHAR)

Question: Which Date has a Builder of alco schenectady, and a Number larger than 835, and Works number of 56566?
SQL: SELECT AVG(date) FROM table_name_19 WHERE builder = "alco schenectady" AND number > 835 AND works_number = 56566