CREATE TABLE table_6997 ("Locomotive" TEXT, "Serial No" TEXT, "Entered service" TEXT, "Owner" TEXT, "Operator" TEXT, "Livery" TEXT, "Gauge" TEXT, "Status" TEXT)

What is the Serial Number of the Locomotive that Entered Service October 1989?
SELECT "Serial No" FROM table_6997 WHERE "Entered service" = 'october 1989'
SELECT "Serial No" FROM "table_6997" WHERE "Entered service" = 'october 1989'