Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_6997 ( "Locomotive" text, "Serial No" text, "Entered service" text, "Owner" text, "Operator" text, "Livery" text, "Gauge" text, "Status" text )

### Question ###
What is the Serial Number of the Locomotive that Entered Service October 1989?

### Accurate SQL ###

SELECT "Serial No" FROM table_6997 WHERE "Entered service" = 'october 1989'