Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_name_26 (serial_no VARCHAR, entered_service VARCHAR)
### Question:
What is the Serial Number of the Locomotive that Entered Service October 1989?
### Answer:
SELECT serial_no FROM table_name_26 WHERE entered_service = "october 1989"